Method and apparatus for verifying and characterizing data retention time in a DRAM using built-in test circuitry

• US 6,272,588 B1
• Filed: 05/30/1997
• Issued: 08/07/2001
• Est. Priority Date: 05/30/1997
• Status: Expired due to Term

• Current Assignee: VLSI Technology LLC (FIG LLC (d/b/a Fortress Investment Group LLC))
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Giles, Grady Lawrence, Johnston, Thomas Kevin, Atwell, William Daune
• Primary Examiner(s): Peikari, Behzad James
• Application Number:
  US08/865,510
• Time in Patent Office: 1,530 Days
• Field of Search: 711/105, 711/106, 711/167, 711/201, 711/210, 711/217, 711/203, 326/37, 326/38, 361/684, 365/203, 365/201, 365/222, 365/225.7, 714/718, 714/743, 714/721
• US Class Current: 711/106
• CPC Class Codes: G11C 11/401 forming cells needing refreshing or charge regeneration, i.e. dynamic cells
• CPC Class Codes: G11C 29/028 with adaption or trimming of parameters
• CPC Class Codes: G11C 29/14 Implementation of control logic, e.g. test mode decoders
• CPC Class Codes: G11C 29/50 Marginal testing, e.g. race, voltage or current testing
• CPC Class Codes: G11C 29/50012 of timing
• CPC Class Codes: G11C 29/50016 of retention

Abstract

A BIST controller (112) and methodology uses the DRAM controller (108) refresh signals to test the data retention characteristics of a DRAM memory array (132). The BIST controller blocks a fraction of the refresh cycles generated by the DRAM controller to provide a margin of confidence above the DRAM'"'"'s specified retention time. The BIST controller is especially suited to embedded applications in which access to the memory is indirect and to applications in which the memory system is modular. The invention may also be used to characterize the actual retention time of a particular DRAM allowing the system to optimize the DRAM'"'"'s refresh interval.

Claims

1. A method for verifying a data retention time in a dynamic random access memory (DRAM) using built-in test circuitry, comprising the steps of:

providing a DRAM having a plurality of rows of memory cells, each row periodically requiring a refresh operation to retain valid data;

providing refresh control circuitry coupled to the DRAM, the refresh control circuitry providing at least one signal to the DRAM which initiates the refresh operation of the DRAM, the DRAM requiring every row to periodically receive a refresh operation to satisfy a data retention time specification for the DRAM; and

providing built-in test circuitry coupled to the refresh control circuitry and the DRAM, the built-in test circuitry dynamically determines a limit for reliable data retention of the DRAM by testing the DRAM with application of variable refresh rates until at least a first failure of data retention is encountered and scaling a rate of memory refresh at which the DRAM is being refreshed, the scaling being between a normal rate at which the refresh control circuitry initiates a refresh operation and a rate equal to the product of;

2. The method of claim 1 further comprising the step of:

operating the built-in test circuitry as a finite state machine which is clocked by a clock signal having a frequency which does not affect or modify a test interval period for testing the data retention time of the DRAM.

3. The method of claim 1 wherein the step of providing the built-in test circuitry to scale the rate of memory refresh further comprises scaling to inhibit refreshing every N out of (M+N) refresh operations.

4. The method of claim 1 wherein the step of scaling the rate of memory refresh further comprises scaling to inhibit N times in every (M+N) refresh operations and further comprises using a programmable counter in which N and M are programmable by a user and the programmable counter implements, at least in part, the inhibiting.

5. The method of claim 1 wherein the step of providing the built-in test circuitry to scale the rate of memory refresh comprises inhibiting N times out of every (M+N) refresh operations and further comprises utilizing a counter to count a number of refresh operations.

6. The method of claim 1 wherein the step of enabling the built-in test circuitry to dynamically determine a limit for reliable data retention of the DRAM further comprises providing circuitry to report a limit to a controller of the refresh control circuitry in order to change a refresh interval to optimize performance of the DRAM.

7. A memory with data retention test capability, comprising:

a dynamic random access memory (DRAM) array having rows and columns of memory cells, each one of the rows being specified to be refreshed within a predetermined refresh period;

a DRAM controller coupled to the DRAM, the DRAM controller providing refresh signals to the DRAM to control timing of refreshing of data of each one of the rows of the DRAM; and

test circuitry coupled to the DRAM, the test circuitry modifying the timing of refreshing of data by the DRAM controller with circuitry which selectively nullifies every N out of (M+N) DRAM refresh operations, where N is less than M, and N and M are integers, and thereby scales the DRAM refresh relative to the predetermined refresh period.

8. The memory of claim 7 wherein the test circuitry further comprises:

a programmable counter coupled to the DRAM, the programmable counter counting values for N and M, where N is provided by a user of the memory.

9. A memory test circuit for testing data retention of a DRAM, comprising:

test circuitry which electrically monitors data refresh signals provided to the DRAM, the test circuitry selectively nullifying every N out of (M+N) data refresh signals, where N is less than M, and N and M are integers, the test circuitry verifying a data retention time characteristic of the DRAM, the test circuitry being clocked by a clock signal having a frequency which does not affect or modify a test interval period for testing the data retention of the DRAM.

10. The memory test circuit of claim 9 wherein the test circuitry further comprises:

a programmable circuit for permitting a user to program a value for N.

11. The memory test circuit of claim 9 wherein the test circuitry further comprises:

a characterization circuit which provides a plurality of values of N from zero to (M−

1), thereby permitting the memory test circuit to characterize limits of reliable data retention of the DRAM.