Scan testing digital logic with differing frequencies of system clock and test clock

• US 5,717,702 A
• Filed: 11/19/1996
• Issued: 02/10/1998
• Est. Priority Date: 03/14/1995
• Status: Expired due to Fees

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Electronics Corporation (Rtx Corporation)
• Inventors: Farwell, William D., Stokes, Robert L.
• Primary Examiner(s): Beausoliel, Jr., Robert W.
• Assistant Examiner(s): VALES, PHILLIP
• Application Number:
  US08/751,379
• Time in Patent Office: 448 Days
• Field of Search: 371/22.31, 371/22.36, 371/22.32, 371/22.35, 371/22.33, 371/22.34, 395/183.06
• US Class Current: 714/730
• CPC Class Codes: G01R 31/318536 Scan chain arrangements, e.g. connections, test bus, analog signals
• CPC Class Codes: G01R 31/318552 Clock circuits details

Abstract

A boundary scan test circuit that includes Y scan flip-flops serially connected in a sequence from a first scan flip-flop to a Y^th scan flip-flop and clocked with a system clock signal, and circuitry for providing scan input data to the first scan flip-flop synchronously with a test clock signal and for receiving scan output data from the Y^th scan flip-flop synchronously with the test clock signal, wherein the test clock signal and the system clock signal have a test clock period to system clock period ratio that is equal to any fixed integer ratio M.

Claims

1. A test circuit comprising:

Y scan flip-flops each having an input and an output, said Y scan flip-flops being connected serially in a sequence from a first scan flip-flop to an Y^th scan flip-flop whereby the output of each of the first through (Y-1)^th flip-flops is connected to the input of a scan flip-flop that is immediately next in sequence, said Y scan flip-flops being clocked by a system clock signal;

feedback means connected between the output of the Y^th scan flip-flop and the input of the first scan flip-flop for recirculating data in the scan flip-flops at the frequency of the system clock signal and for providing input data to the first scan flip-flop at the frequency of a test clock signal, wherein the ratio of the period of the test clock to the period of the system clock is equal to an integer M, said feedback means comprising;

synchronizing means responsive to the system clock signal and the test clock signal for generating a synchronizing signal that is in a first logical state during every M^th active transition of the system clock signal and in a second logical state between occurrences of said first logical state;

serial-to-parallel shift register means having L registers each having an input and an output, said L registers being connected serially in a sequence from a first register to an L^th register whereby the output of each of the first through (L-1)^th register is connected to the input of a register that is immediately next in sequence, said L registers being clocked by the system clock signal;

first selection means having inputs connected to the outputs of said serial-to-parallel shift register and the output of said Y^th scan flip-flop for providing an output that is a replica of a selected one of said inputs;

control means connected to said first selection means for providing control signals to select said selected one of said inputs, wherein a length of said shift register means is programmably set by said control means to thereby provide a variable extension of said sequence of scan flip-flops;

second selection means having a first input connected to the output of said first selection means, a second input for receiving scan input data, and an output connected to the input of said first scan flip-flop, said selection means responsive to said synchronizing signal for providing a selection means output that comprises (a) a replica of the scan input when said synchronizing signal is in said first logical state, and (b) a replica of the output of said first selection means when said synchronizing signal is in said second logical state; and

an output flip-flop clocked by the system clock and having an input connected to the output of said first selection means, said output flip-flop being periodically enabled pursuant to said test clock signal when said synchronization signal is in its first logical state.