Integrated circuit I/O using a high performance bus interface

• US 5,809,263 A
• Filed: 12/09/1996
• Issued: 09/15/1998
• Est. Priority Date: 04/18/1990
• Status: Expired due to Term

• Current Assignee: Rambus Inc.
• Original Assignee: Rambus Inc.
• Inventors: Horowitz, Mark, Farmwald, Michael
• Primary Examiner(s): AUVE, GLENN ALLEN
• Application Number:
  US08/762,139
• Time in Patent Office: 645 Days
• Field of Search: 395/309, 395/280, 395/306, 395/307, 395/497.01, 395/497.03, 395/308, 711/170, 711/171, 711/211, 711/212
• US Class Current: 710/305
• CPC Class Codes: G06F 11/006 Identification G06F11/2289 takes precedence
• CPC Class Codes: G06F 11/1048 using arrangements adapted for a specific error detection or correction feature
• CPC Class Codes: G06F 12/0215 with look ahead addressing means
• CPC Class Codes: G06F 12/0661 and decentralised selection
• CPC Class Codes: G06F 12/0684 with feedback, e.g. presence or absence of unit detected by addressing, overflow detection
• CPC Class Codes: G06F 13/1605 based on arbitration arbitration in handling access to a common bus or bus system G06F13/36
• CPC Class Codes: G06F 13/161 with latency improvement
• CPC Class Codes: G06F 13/1678 using bus width
• CPC Class Codes: G06F 13/1689 Synchronisation and timing concerns synchronisation on a memory bus G06F13/4234
• CPC Class Codes: G06F 13/1694 Configuration of memory controller to different memory types
• CPC Class Codes: G06F 13/376 using a contention resolving method, e.g. collision detection, collision avoidance
• CPC Class Codes: G06F 13/4239 with asynchronous protocol
• CPC Class Codes: G11C 11/4076 Timing circuits for regeneration management G11C11/406
• CPC Class Codes: G11C 11/4096 Input/output [I/O] data management or control circuits, e.g. reading or writing circuits, I/O drivers or bit-line switches
• CPC Class Codes: G11C 2207/105 Aspects related to pads, pins or terminals
• CPC Class Codes: G11C 2207/108 Wide data ports
• CPC Class Codes: G11C 29/88 with partially good memories
• CPC Class Codes: G11C 5/00 Details of stores covered by group G11C11/00
• CPC Class Codes: G11C 5/04 Supports for storage elements , e.g. memory modules; Mounting or fixing of storage elements on such supports
• CPC Class Codes: G11C 5/06 Arrangements for interconnecting storage elements electrically, e.g. by wiring
• CPC Class Codes: G11C 5/063 : Voltage and signal distribution in integrated semi-conductor memory access lines, e.g. word-line, bit-line, cross-over resistance, propagation delay
• CPC Class Codes: G11C 5/066 : Means for reducing external access-lines for a semiconductor memory clip, e.g. by multiplexing at least address and data signals
• CPC Class Codes: G11C 7/1006 : Data managing, e.g. manipulating data before writing or reading out, data bus switches or control circuits therefor
• CPC Class Codes: G11C 7/1012 : Data reordering during input/output, e.g. crossbars, layers of multiplexers, shifting or rotating
• CPC Class Codes: G11C 7/1045 : Read-write mode select circuits
• CPC Class Codes: G11C 7/1051 : Data output circuits, e.g. read-out amplifiers, data output buffers, data output registers, data output level conversion circuits
• CPC Class Codes: G11C 7/1057 : Data output buffers, e.g. comprising level conversion circuits, circuits for adapting load
• CPC Class Codes: G11C 7/1066 : Output synchronization
• CPC Class Codes: G11C 7/1069 : I/O lines read out arrangements
• CPC Class Codes: G11C 7/1072 : for memories with random access ports synchronised on clock signal pulse trains, e.g. synchronous memories, self timed memories
• CPC Class Codes: G11C 7/1078 : Data input circuits, e.g. write amplifiers, data input buffers, data input registers, data input level conversion circuits
• CPC Class Codes: G11C 7/1084 : Data input buffers, e.g. comprising level conversion circuits, circuits for adapting load
• CPC Class Codes: G11C 7/22 : Read-write [R-W] timing or clocking circuits; Read-write [R-W] control signal generators or management
• CPC Class Codes: G11C 7/222 : Clock generating, synchronizing or distributing circuits within memory device
• CPC Class Codes: G11C 7/225 : Clock input buffers
• CPC Class Codes: G11C 8/00 : Arrangements for selecting an address in a digital store for stores using transistors G11C11/407, G11C11/413
• CPC Class Codes: Y02D 10/00 : Energy efficient computing, e.g. low power processors, power management or thermal management

Abstract

A memory subsystem for storing and retrieving data. At least one memory device Includes a bus Interface. The memory device has at least one memory section comprised of a plurality of memory cells. The bus interface of the at least one memory device couples the memory device to a bus. The bus comprises a group of controlled impedance transmission lines for carrying substantially all information necessary for a single memory device to receive a transaction request, including a memory transaction request, and for carrying substantially all information necessary for a single memory device to respond to the transaction request. The number of signaling lines is substantially less than the number of bits in the information necessary to request a memory transaction to store or retrieve data from the memory cells. Memory device selection information is time-multiplexed on the bus with other memory transaction request information.

Claims

1. A memory subsystem for storing and retrieving data, comprising:

(1) at least one memory device that includes a bus interface, the memory device having at least one memory section comprised of a plurality of memory cells; and

(2) a bus, wherein the bus interface of the at least one memory device couples the memory device to the bus, wherein the bus comprises a group of controlled impedance transmission lines for carrying substantially all information necessary for a single memory device to receive a transaction request, including a memory transaction request, and for carrying substantially all information necessary for a single memory device to respond to the transaction request;

wherein the number of signaling lines is substantially less than the number of bits in the information necessary to request a memory transaction to store or retrieve data from the memory cells; and

wherein memory device selection information is time-multiplexed on the bus with other memory transaction request information.

2. The memory subsystem of claim 1, wherein the transmission lines have substantially the same electrical characteristics.

3. The memory subsystem of claim 1, wherein the at least one memory device further comprises a device identification register, the contents of which are used to uniquely identify the at least one memory device among a plurality of memory devices connected to the bus;

wherein the transaction request includes a field specifying the memory device identification number; and

wherein the memory device responds to the transaction request when its identification number matches the identification number in the transaction request.

4. The memory subsystem of claim 3, further comprising configuration means for loading an identification value into the identification register.

5. The memory subsystem of claim 1, wherein the at least one memory device further comprises at least one address range register coupled to the at least one memory section, the contents of the address range register determining a range of addresses that corresponds to the memory section;

wherein the memory transaction request includes address information; and

wherein the at least one memory device responds to the memory transaction request when the address information falls within the range of addresses specified by the contents of the address range register.

6. The memory subsystem of claim 1, wherein the at least one memory device further comprises at least one access time register whose contents indicates a delay time after which the memory device must respond to a transaction request.

7. The memory subsystem of claim 6, further comprising a clock signaling line, wherein the delay time is determined by a number of clock cycles of the clock signaling line.

8. The memory subsystem of claim 1, wherein the at least one memory device further comprises at least one control register containing information that affects the operation of the memory device;

wherein the transaction request includes information designating a register transaction to be performed by the memory device and address information for selecting one of the control registers; and

wherein the memory device responds to the transaction request by transferring data information between the transmission lines and the selected one of the control registers via the bus interface.

9. The memory subsystem of claim 1, wherein the bus interface has an input coupled to receive an external clock signal having at least two phases;

and wherein the bus interface further comprises;

(i) a plurality of I/O pads;

(ii) internal clock generation circuitry for receiving the external clock signal and generating at least two internal clocks, each internal clock having a phase associated with one of the phases of the external clock signal; and

(iii) a plurality of input receiver groups, each group comprising at least two receivers, coupled to each I/O pad and the internal clocks for receiving information present during each phase of the external clock from the bus and transferring the bus information from the plurality of I/O pads to a plurality of internal input lines coupled to the receiver groups.

10. The memory subsystem of claim 1, wherein the bus interface includes a bus driver for each of the transmission lines, the bus driver being coupled between a transmission line and ground and operating as a current source for current having a predetermined value;

wherein a first voltage is present on the transmission line when the current source is not conducting current and a second voltage is present on the transmission line when the current source is conducting current of the predetermined value; and

wherein the difference between the first voltage and second voltage is proportional to the product of the predetermined value of the current source and the impedance of the transmission line.

11. The memory subsystem of claim 10, wherein the first voltage is determined by a termination resistor coupled between a termination voltage and one end of the transmission line.

12. The memory subsystem of claim 1, wherein the bus interface includes internal clock generation circuitry receiving an external clock signal and generating a delay-locked internal clock having a desired phase difference between the external clock signal and the internal clock signal.

13. The memory subsystem of claim 12, wherein the internal clock generation circuitry comprises:

(1) a receiver for receiving the external clock signal and generating the internal clock;

(2) a delay line for adjustably delaying the internal clock to produce a delayed internal clock signal;

(3) means for detecting the phase difference between the delayed internal clock signal and the external clock signal and generating an error signal proportional to the phase difference; and

(4) means for actively and continuously adjusting the delay line based on the error signal so that the delayed internal clock signal has the desired phase difference relative to the external clock signal.

14. The memory device of claim of 1,

wherein the bus interface has an input coupled to receive an external clock signal having at least two phases;

and

wherein the bus interface comprises;

a plurality of I/O pads;

internal clock generation circuitry for receiving the external clock signal and generating at least two internal clocks, each internal clock having a phase associated with one of the phases of the external clock signal;

a plurality of bus drivers, each coupled to an I/O pad and having an input for receiving date to be transferred to the bus during each phase of the external clock signal; and

a plurality of multiplexers coupled to the internal clocks and to each bus driver, the multiplexer selectively coupling data information in response to the phases of the internal clocks to the input of the bus drivers.

15. A method of communicating over a bus in a memory subsystem, the method comprising the steps of:

(1) a master transmitting on the bus a transaction request,

wherein the bus comprises a group of signaling lines that carry substantially all information necessary for a single memory device to receive a transaction request, including a memory transaction request,

wherein the signaling lines carry substantially all Information necessary for a single memory device to respond to the transaction request, and

wherein the number of signaling lines is substantially less than the number of bits in the memory transaction request;

wherein memory device selection information is time-multiplexed on the bus with other memory transaction request information;

(2) the memory device receiving the transaction request transmitted on the signaling lines;

(3) the memory device determining whether it should respond to the transaction request; and

(4) if the memory device has determined that it should respond to the transaction request, then the memory device responding to the transaction request by receiving data information sent by the master on the signaling lines when receiving data information is necessary to perform a first type of transaction indicated in the transaction request, or by transmitting data information to the master on the signaling lines when transmitting data information is necessary to perform a second type of transaction indicated in the transaction request.

16. The method of claim 15, wherein the transaction request includes address information, and wherein the step of the memory determining whether it should respond to the transaction request comprises determining whether the memory device should respond to the address information in the transaction request.

17. The method of claim 15, wherein the memory device comprises a device identification register whose contents uniquely identify the memory device;

wherein the transaction request further includes device identification information; and

wherein the step of the memory determining whether it should respond to the transaction request comprises determining whether the device identification information in the transaction request matches the device information in the device identification register.

18. The method of claim 17, wherein the device identification information in the transaction request designates all the memory devices coupled to the bus so that all of the memory devices respond to the transaction request.

19. The method of claim 15, wherein the memory device has a plurality of access time registers each loaded with an access time, the access time being a delay time after the receipt of the transaction request at which the memory device must respond to the transaction indicated in the transaction request;

wherein the transaction request further comprises access time information designating an access time register; and

wherein the step of the memory device responding to the transaction request comprises responding at a time specified by the access time register designated by the access time information in the transaction request.

20. The method of claim 19, wherein the master issues transaction requests that designate access times for responses so that possession of the bus is at all times controlled and scheduled by the master to increase bus utilization.

21. The method of claim 15, wherein the transaction request is a register transaction request comprising address information and direction information;

wherein the memory device includes a plurality of control registers, the address information specifying one of the control registers; and

wherein the step of responding to the transaction specified in the transaction request comprises transmitting information from one of the control registers according to the direction information or receiving information from one of the control registers according to the direction information.

22. The method of claim 21, wherein the control register is an access time register, the contents of which determine a delay time at which the memory device must respond to a transaction request.

23. The method of claim 21, wherein the control register is an identification register, the contents of which uniquely identify the memory device.

24. The method of claim 21, wherein the control register is an address range register, the contents of which describe a range of addresses for which the memory device must respond to the transaction request.

25. A memory device, comprising:

(1) at least one memory section comprised of a plurality of memory cells;

(2) a bus interface for coupling the memory section to a bus having a group of signaling lines, wherein the number of signaling lines is substantially less than the number of bits in the information necessary to request a memory transaction to store or retrieve data from the memory cells;

wherein the signaling lines carry substantially all information necessary for a single memory device to receive the memory transaction request;

wherein the signaling lines carry substantially all information necessary for a single memory device to respond to the memory transaction request;

wherein memory device selection information is time-multiplexed on the bus with other memory transaction request information; and

wherein the signaling lines comprise controlled impedance transmission lines.

26. The memory device of claim 25, wherein the transmission lines have substantially the same electrical characteristics.

27. The memory device of claim 25, wherein the bus interface includes a bus driver for each of the signaling lines, wherein each bus driver is coupled between a signaling line and ground and operates as a current source for current having a predetermined value;

wherein a first voltage is present on the signaling line when the current source is not conducting current;

wherein a second voltage is present on the signaling line when the current source is conducting current of the predetermined value;

wherein the difference between the first and second voltage is proportional to the product of the predetermined value of the current of the current source and the impedance of the signaling line; and

wherein the first voltage is determined by a termination resistor coupled between a termination voltage and one end of the signaling line.

28. An apparatus comprising:

(1) at least two semiconductor devices;

(2) a group of signaling lines to which the semiconductor devices are coupled,

wherein the number of signaling lines is substantially less than the number of bits in the information necessary to request a transaction to be performed by a single semiconductor device,

wherein the signaling lines carry substantially all information necessary for a single semiconductor device to receive the transaction request,

wherein the signaling lines carry substantially all information necessary for a single semiconductor device to respond to the transaction request,

wherein the signaling lines comprise transmission lines having substantially the same electrical characteristics and the transmission lines are of a controlled impedance,

wherein the signals on the signaling lines are created from current source drivers; and

(3) at least one clock line for transmission a pair of clock signals to be received by the semiconductor devices,

wherein the clock signals have a clock rate a short cycle time, and a clock cycle having a first phase and a second phase,

wherein information necessary to request and to respond to a transaction is placed on the group of signaling lines in response to the start of the first phase, and

wherein information necessary to request and to respond to a transaction is placed on the group of signaling lines in response to the start of the second phase so that a data information rate is twice the clock rate.

29. An apparatus for storing and retrieving data, the apparatus comprising:

(1) a multiline bus for transmitting address information, control information, and data, wherein the multiline bus has a total number of lines less than a total number of bits in any single address and wherein the control information includes information for selecting memories;

(2) a master coupled to the multiline bus for (A) initiating a read operation by placing on the multiline bus a read request packet comprising (i) read control information comprising a read device identifier, a read cycle type, and a maximum data transfer length indicator and (ii) read address information comprising a read starting address, and for (B) initiating a write operation by placing on the multiline bus a (i) write request packet comprising (a) write control information comprising a write device identifier and a write cycle type and (b) write address information comprising a write starting address and (ii) write data

(3) a first memory coupled to the multiline bus for responding to the read request packet from the master by placing onto the multiline bus data returned from a location within the first memory beginning at the read starting address if the read identifier matches an identifier of the first memory;

(4) a second memory coupled to the multiline bus for responding to the write request packet from the master by storing in the second memory at the write starting address the write data if the write device identifier matches an identifier of the second memory.

30. The apparatus of claim 29, further comprising a clock coupled to the master, the first memory, and the second memory.

31. The apparatus of claim 29, wherein the multiline bus does not have dedicated memory select lines for selecting memories.

32. The apparatus of claim 29, wherein the master is a central processing unit.

33. An apparatus for high-speed access to blocks of data according to a synchronous, split transaction, block-oriented protocol, the apparatus comprising:

(1) a multiline bus for transmitting address information, control information, and blocks of data, wherein the multiline bus has a total number of lines less than the total number of bits in a single address, wherein the control information includes information for selecting memories;

(2) a clock running at a clock rate;

(3) a master coupled to the clock and to the multiline bus for (A) initiating a clocked read operation of a first block of data by placing on the multiline bus a read request packet comprising read address and control information and for (B) initiating a clocked write operation of a second block of data by placing on the multiline bus a (I) write request packet comprising write address and control information and the (ii) second block of data;

(4) a first memory coupled to the clock and to the multiline bus for responding to the read request packet from the master by placing onto the multiline bus in a clocked manner the first block of data returned from a location within the first memory if the read request packet identifies the first memory;

(5) a second memory coupled to the clock and to the multiline bus for responding to the write request packet from the master by storing in the second memory the second block of data received from the multiline bus in a clocked manner if the write request packet identifies the second memory.

34. An apparatus for high-bandwidth data transfer, the apparatus comprising:

(1) a multiline bus for transmitting address information, control information, and data, wherein the control information includes information for selecting memories without the use of dedicated memory select lines;

(2) a master coupled to the multiline bus for initiating a read operation by placing on the multiline bus a read request packet and for initiating a write operation by placing on the multiline bus a write request packet followed by write data;

(3) a first memory coupled to the multiline bus via bus interface built into the first memory and residing on a single edge of the first memory, wherein the first memory responds to the read request packet from the master by placing on the multiline bus data returned from a location within the first memory if the read request packet identifies the first memory;

(4) a second memory coupled to the multiline bus via a bus interface built into the second memory and residing on a single edge of the second memory, wherein the second memory responds to the write request packet from the master by storing in the second memory data received from the multiline bus if the write request packet identifies the second memory.

35. The apparatus of claim 34, wherein the bus interface of the first memory and the bus interface of the second memory each includes an electrical interface, address comparison registers, timing registers, and a memory array access path.

36. A method for retrieving data, comprising the steps of:

(1) having a master initiate a read operation by placing onto a multiline bus a read request packet comprising a read device identifier, a read cycle type, a maximum transfer length indicator, and a read starting address;

(2) having a first memory of a plurality of memories coupled to the multiline bus respond to the read request packet from the master by placing onto the multiline bus data returned from a location within the first memory beginning at the read starting address if the read device identifier matches an identifier of the first memory.

37. A method for storing data, comprising the steps of:

(1) having a master initiate a write operation by placing onto a multiline bus (A) a write request packet comprising write device identifier, a write cycle type, and a write starting address and (B) write data; and

(2) having a first memory of a plurality of memories coupled to the multiline bus respond to the write request packet from the master by storing in the first memory at the write starting address the write data if the write device identifier matches an identifier of the first memory.

38. The method of claim 37, further comprising the step of:

having the plurality of memories including the first memory respond to the write request packet from the master by storing in each of the plurality of memories at the write starting address the write data if the write device identifier matches a broadcast identifier.

39. A method for storing and retrieving data, comprising the steps of:

(1) having a master attempt a read operation by placing onto a multiline bus a read request packet comprising a read device identifier, a read cycle type, a maximum transfer length indicator, and a read starting address;

(2) having the master attempt a write operation by placing onto a multiline bus (A) a write request packet comprising a write device identifier, a write cycle type, and a write starting address and (B) write data;

(3) if the read device identifier or write device identifier matches an identifier of a first memory of a plurality of memories coupled to the multiline bus and the first memory cannot respond to the read request packet or the write request packet, then having the first memory place onto the multiline bus a retry message request that the master retry a request.

40. An apparatus for high-bandwidth data transfer, the apparatus comprising:

(1) a multiline bus for transmitting address information, control information, and data, wherein the multiline bus has a total number of lines less than a total number of bits in any single address, wherein the control information includes information for selecting memories;

(2) a master coupled to the multiline bus for (A) initiating a read operation by placing on the multiline bus a read request packet comprising read address and control information and for (B) initiating a write operation by placing on the multiline bus (i) a write request packet comprising write address and control information and (ii) write data;

(3) a first memory coupled to the multiline bus for responding to the read request packet from the master by placing onto the multiline bus data returned from a location within the first memory if the read request packet identifies the first memory;

(4) a second memory coupled to the multiline bus for responding to the write request packet from the master by storing in the second memory data received from the multiline bus if the write request packet identifies the second memory; and

(5) timing circuitry for ensuring that the first memory places data onto the multiline bus at a predetermined time.

41. An apparatus comprising;

(1) a master coupled to a bus for initiating a response by placing on the bus a request packet; and

(2) a slave coupled to the bus, wherein the slave includes an access-time register that stores a value that determines a timing of a response by the slave to the request packet.

42. The apparatus of claim 41, wherein the access-time register is coupled to the bus and wherein the value stored in the access-time register is settable by the master via a bus transaction.

43. The apparatus of claim 41, wherein the value stored in the access time register is hardwired.

44. An apparatus comprising:

(1) a slave coupled to a bus, wherein the slave includes a plurality of selectable access-time registers storing a plurality of timing values, wherein a timing value stored by an access-time register selected from the plurality of access-time registers determines a timing of a response by the slave to a request packet;

(2) a master coupled to a bus for initiating the response by the slave by placing on the bus the request packet, wherein the request packet includes code for selecting one of the plurality of access-time registers.

45. The apparatus of claim 44, wherein the plurality of access-time registers are coupled to the bus and wherein the plurality of timing values are settable by the master via a bus transaction.

46. The apparatus of claim 44, wherein one of the plurality of timing values is hardwired and another of the plurality of timing values is settable by the master via a bus transaction.

47. A memory device, comprising:

(1) at least one memory section comprised of a plurality of memory cells;

(2) a bus interface for coupling the memory section to a bus comprised a group of signaling lines, wherein the number of signaling lines is substantially less than the number of bits in a memory transaction request to store or retrieve data from the memory cells;

wherein the signaling lines carry substantially all Information necessary for a single memory device to receive the memory transaction request;

wherein the signaling lines carry substantially all information necessary for a single memory device to respond to the memory transaction request;

wherein substantially all information necessary for a single memory device to receive the memory transaction request includes address information and control information;

wherein the control information includes memory device selection information; and

wherein the control information is time multiplexed with the address information over the group of signaling lines.

48. A method of communicating over a bus to a memory device, the bus comprising a group of signaling lines, the method comprising the steps of:

(1) a master transmitting on the bus a transaction request, the bus carrying substantially all address, data, and control information needed by the memory device to receive and respond to the transaction request and having substantially fewer signaling lines than the number of bits in the address information, the control information including memory device selection information and being time-multiplexed with address information over the group of signaling lines;

(2) the memory device receiving the transaction request transmitted on the signaling lines; and

(3) the memory device responding to the received transaction request according to the information in the transaction request.

49. An apparatus comprising:

(1) at least two semiconductor devices, each having a bus interface; and

(2) a bidirectional bus comprising a group of signaling lines and coupling to the bus Interfaces of the semiconductor devices, wherein the number of signaling lines is substantially less than the number of bits in the information necessary for one device to request a transaction to be performed by the other device, wherein the signaling lines carry substantially all information necessary for the semiconductor device to receive and respond to the transaction request, and wherein the transaction request includes device selection information time-multiplexed on the bus with other transaction request information.

50. A memory device comprising:

at least one memory section comprised of a plurality of memory cells;

a synchronous bus interface coupling the memory section to a bus comprising a group of signaling lines, the synchronous bus interface having an input coupled to receive an external clock signal and receiving a transaction request on the bus synchronously with the external clock signal, the synchronous bus interface receiving or transferring data information on the bus synchronously with the external clock signal in response to the transaction request; and

wherein the bus carries substantially all address, data, and control information needed by the memory device to receive and respond to the transaction request and the bus has substantially fewer bus lines than the number of bits in the address information.

51. The memory device of claim 50, wherein the control information is time-multiplexed with the address information over the group of signaling lines.

52. A memory device comprising:

at least one dynamic memory section comprised of a plurality of memory cells; and

a synchronous bus interface coupling the memory section to a bus comprising a group of signaling lines, the synchronous bus interface having an input coupled to receive an external clock signal, wherein the interface receives control and address Information on the bus synchronously with the external clock signal, the control information being time-multiplexed with address information over the group of signaling lines and wherein the bus has substantially fewer bus lines than the number of bits in the address information.

53. The memory device of claim 52, wherein the external clock signal has a cycle with at least two phases, the transaction request comprising information that is valid on each phase of the external clock signal.

54. The memory device of claim 52, the external clock signal having a cycle with at least two phases, the memory device response comprising information that is valid on each phase of the external clock signal.

55. The memory device of claim 52, wherein the bus interface includes:

internal clock generation circuitry receiving the external clock signal and generating a delay-locked internal clock with a desired phase difference between the external clock signal and the internal clock, wherein the internal clock generation circuitry comprises;

a clock receiver receiving the external clock signal and generating an internal clock;

a delay line for generating a delayed internal clock from the internal clock in response to a delay adjustment signal;

circuitry for determining a phase difference signal between the delayed internal clock and external clock signal; and

circuitry for generating the delay adjustment signal to adjust the delay line in response to the phase difference signal so that the delayed internal clock becomes delay-locked with the desired phase difference relative to the external clock.

56. The memory device of claim 52, wherein the memory device further comprises at least one access time register whose contents indicates a delay time after which the memory device must respond to a transaction request.

57. An apparatus comprising:

at least two semiconductor devices each having a bus interface; and

a bidirectional bus comprising a group of signaling lines and coupling to the bus interfaces of the semiconductor devices;

wherein the number of signaling lines is substantially less than the number of bits in the information necessary for one device to request a transaction to be performed by the other device;

wherein the signaling lines carry substantially all information necessary for the semiconductor device to receive and respond to the transaction request; and

wherein the transaction request includes device selection information time-multiplexed on the bus with other transaction request information.