Fully Scan-Set Testable Embedded Edge-Triggered Dual D and J-K Flip-Flops Through Testing as Inverter Strings

1. An extended and expanded improved testing method to that scan-set testing of an edge-triggered flip-flop which, by the interconnection of some of the NAND gate logic elements thereof said flip-flop, does exercise said elements as a race-free bistable flip-flop during scan-set testing responsively to an active scan-set first test enablement signal, called TEST 1, plus active test clock signals, called SS CLK A and SS CLK B, to shift the scan-set test data signal input, called SS DATA IN, to a scan-set test data signal output, called SS DATA OUT, said improved testing method, to the end that remaining ones of the NAND gate logic elements of said edge-triggered flip-flop should also be tested, comprising: identifying those serially interconnected first NAND gates of the edge-triggered flip-flop logic structure which were disabled, and were therefore untestable, by said active condition of said first test enablement signal, said signal TEST 1, during said scan set-testing; causing said first test enablement signal, said signal TEST 1, to assume the inactive condition in partial enablement of those said serially interconnected first NAND gates which had been previously disabled during said scan-set testing; identifying those second NAND gates which are cross-coupled with any one or ones of said serially interconnected first NAND gates, said cross-coupling meaning that when said second NAND gates are operative then the active signal outputs therefrom will be received at the signal inputs to any one or ones of said serially interconnected first NAND gates; applying the active condition of a second test enablement signal, called TEST 2, in order to disable said second NAND gates; gating in a third NAND gate the active scan-set test data signal input, said SS DATA IN, with said active second test enablement signal, said TEST 2, in order to produce a second-test stimuli signal; applying said second-test stimuli signal in partial enablement of the first of said serially interconnected first NAND gates; allowing an actively cycling system clock signal, called SYST CLK, received at said serially interconnected first NAND gates, to clock said second-test stimuli signal through both said serially interconnected first NAND gates; merging said clocked second-test stimuli signal upon the output of the last of said serially interconnected first NAND gates into, and becoming as, said test data signal output, said SS DATA OUT; whereby said serially interconnected first NAND gates which were disabled, and therefore untestable, during said scan-set testing are enabled and interconnected between said test data signal input, said SS DATA IN, and said test data signal output, said SS DATA OUT, during the active condition of said second test enablement signal, said TEST 2, and said actively cycling system clock signal, said SYST CLK; whereby said serially interconnected first NAND gates are tested.

2. The extended and expanded improved testing method of claim 1 further comprising: observing the signal propagation delay between said test data signal input, said SS DATA IN, and said test data signal output, said SS DATA OUT, as indicative of both the current drive capacity of said serially interconnected first NAND gates and the impedance experienced by said serially interconnected first NAND gates in the propagation of said test data signal input, said SS DATA IN; wherein said observing of signal propagation delay is useful to assess, as by comparisons between like ones of said edge-triggered flip-flops subject to test, whether there exists abnormally lengthy signal propagation times through said serially interconnected first NAND gates which precludes or may marginally preclude correct reliable operation of the edge-triggered flip-flop structure as embedded.

3. An improvement to the process of scan-set testing an edge-triggered flip-flop embedded in a scan-set shift register, said scan-set testing being in response to, and during, an active first test enablement signal (TEST 1) plus in response to, and during, the active condition of scan-set clock signals (SS CLK A and SS CLK B) in order to shift an input scan-set test data signal (SS DATA IN) to an output scan-set test data signal (SS DATA OUT); and said scan-set testing necessarily disabling, by and during said active condition of said first test enablement signal (TEST 1) both a first input NAND gate (B" or N') and serially interconnected second input NAND gate (C" or O') of said edge-triggered flip-flop which said first (B", or N') and said second (C", O') input NAND gates are identifiable for also uniquely receiving the system clock signal (SYST CLK) in order that, by said disabling by and during said active condition of said first test enablement signal (TEST 1) both said first (B", or N') and said second (C", or O') input NAND gates also receiving said active system clock signal (SYST CLK) will not be satisfied and will thusly not develop output signals which would conflict with said scan-set testing of said edge-triggered flip-flop, said improvement to the process of scan-set testing being to the purpose that said first (B", or N') plus said second (C", or O') input gates may be tested compatibly with, and at a time after, said scan-set teting responsively to said first test enablement signal (TEST 1), said improvement process comprising: causing said first test enablement signal (TEST 1) to assume the inactive condition in partial enablement of said first input NAND gate (B", or N') and said serially interconnected second NAND gate (C", or O'); causing a separate second test enablement signal (TEST 2) to assume the active condition; first applying said active second test enablement signal (TEST 2) to all those NAND gates of the edge-triggered flip-flop, other than said first NAND gate (B", or N ') and said NAND gate (C", or O'), which do receive signals from either said first NAND gate (B", or N') or from said second NAND gate (C", or O'); gating in a third NAND gate (K or T) said scan-set test data signal (SS DATA IN) with said active second test enablement signal (TEST 2) in order to produce a second-test stimuli signal; gating said second-test stimuli through said first NAND gate (B", or N') and then, as the output signal of said first NAND gate, through said serially interconnected second NAND gate (C", or O') during the active condition of said system clock signal (SYST CLK); merging in a plurality of fourth NAND gates the clock-signal-gated second-test stimuli signal derived at the output of said second NAND gate (C", or O') into, and becoming, said output test data signal (SS DATA OUT); wherein a signal path has been created for said input scan-set test data signal (SS DATA IN) through said third NAND gate (K, or T) then through said first NAND gate (B", or N') then through said second NAND gate (C", or O') then through said plurality of fourth NAND gates which do said merging until said signal does become said test data output signal (SS DATA OUT); wherein since said first NAND gate (B", or N') said second NAND gate (C", or O') are within said signal path then both are testable for logical operation by observation of the results of digital signal propagation thereupon said signal path.

4. The improvement to the process of scan-set testing an edge-triggered flip-flop of claim 3 further comprising: observing the signal propagation delay between said input scan-set test data signal (SS DATA IN) and said output scan-set test data signal (SS DATA OUT) as indicative of both the signal drive capacity of said first input NAND gate (B", or N') in combination with said serially interconnected second input NAND gate (said C", or O'), plus the impedance experienced by the signal drive thereof said first input NAND gate (said B", or N') and said serially interconnected second input NAND gate (said C", or O').

5. A digital circuit apparatus in modification of, and with logic augmentations to, the logic structure of an edge-triggered dual D-type flip-flop industry standard part number 74 solely to the end that a first NAND gate (gate B) serially interconnected to a second NAND gate (gate C) within said flip-flop logic structure as both do uniquely receive the system clock signal (SYST CLK) should be selectively exercisable under the control of a test enablement signal (TEST 2) to propagate a test data signal (SS DATA IN) serially through said first NAND gate (said B) and then said second NAND gate (said C) without suffering interference from the remaining NAND gates thereof said flip-flop logic structure, said apparatus comprising: test enablement signal providing means for providing an active test enablement signal (TEST 2); test data signal providing means for providing a test data signal (SS DATA IN); a third NAND gate (gate K) which is enabled by active said test enablement signal (TEST 2) for gating said test data signal (SS DATA IN) to a logically OR'ed interconnection with that signal net of the edge-triggered dual D-type flip-flop industry standard part number 74, which signal net is identifiable as: wherein said first NAND gate (gate B) and said serially interconnected second NAND gate (gate C) of said flip-flop logic structure do uniquely receive the system clock signal (SYST CLK), wherein said first NAND gate (B) does have an output signal connection to said other, second NAND gate (C) while also being cross-coupled to a fourth NAND gate (gate A), then said signal net is between the output of said fourth NAND gate (A) and the input to said cross-coupled first NAND gate (B); said fourth NAND gate (A) plus a fifth NAND gate (D) part of said edge-triggered dual D-type flip-flop industry standard part number 74 logic structure--which said fourth NAND gate (A) and said fifth NAND gate (D) are identifiable as those which are respectively cross-coupled to said first NAND gate (B) and said second NAND gate (C)--modified to accept as an input signal said test enablement signal (TEST 2) and to be disabled by the active condition thereof said test enablement signal (TEST 2); wherein said test data signal (SS DATA IN) as logically OR'ed in interconnection to said signal net is gated through both said first NAND gate (B) and then said second NAND gate (C) serially interconnected thereto upon the enabling occurrence of said system clock signal (SYST CLK) which is received at both said first NAND gate (B) and said second NAND gate (C); thereby since said fourth NAND gate (A) and said fifth NAND gate (D) are disabled by said active condition of such test enablement signal (TEST 2) and thusly not gating of signals, then no signal outputs from either said fourth NAND gate (A) or said fifth NAND gate (D) are interfering with said gating of said test data signal (SS DATA IN) through said first NAND gate (B) and said second NAND gate (C) to which said fourth NAND gate (A) and said fifth NAND gate (D) are respectively cross-coupled.

6. A digital circuit apparatus in modification of, and with logic augmentations to, the logic structure of an edge-triggered J-K flip-flop industry standard part number 109 solely to the end that a first NAND gate (gate N) serially interconnected to a second NAND gate (gate O) within said flip-flop logic structure as both do uniquely receive the system clock signal (SYST CLK) should be selectively exercisable under the control of a test enablement signal (TEST 2) to propagate a test data signal (SS DATA IN) serially through said first NAND gate (said N) and then said second NAND gate (said O) without suffering interference from the remaining NAND gates thereof said flip-flop logic structure, said apparatus comprising: test enablement signal providing means for providing an active test enablement signal (TEST 2); test data signal providing means for providing a test data signal (SS DATA IN); a third NAND gate (gate T1) which is enabled by active said test enablement signal (TEST 2) for gating said test data signal (SS DATA IN) to a logically OR'ed interconnection with that signal net of the edge-triggered J-K flip-flop industry standard part number 109, which signal net is identifiable as: wherein said first NAND gate (gate N) and said serially interconnected second NAND gate (gate O) of said flip-flop logic structure do uniquely receive the system clock signal (SYST CLK), wherein said first NAND gate (N) does have an output signal connection to said other, second NAND gate (O) while also being cross-coupled to a fourth NAND gate (gate M), then said signal net is between the output of said fourth NAND gate (M) and the input to said cross-coupled first NAND gate (N); wherein said fourth NAND gate (M) receives signals from two fifth NAND gates (P and Q) part of said edge-triggered J-K flip-flop industry standard part number 109 logic structure--which said fourth NAND gate (M) and said two fifth NAND gates (P and Q) are identifiable as those which are respectively cross-coupled to said first NAND gate (N) and said second NAND gate (O)--said two fifth NAND gates (P and Q) modified to accept as an input signal said test enablement signal (TEST 2) and to be disabled by the active condition thereof said test enablement signal (TEST 2); wherein said test data signal (SS DATA IN) as logically OR'ed in interconnection to said signal net is gated through both said first NAND gate (N) and then said second NAND gate (O) serially interconnected thereto upon the enabling occurrence of said system clock signal (SYST CLK) which is received at both said first NAND gate (N) and said second NAND gate (O); thereby since said fourth NAND gate (M) and said two fifth NAND gates (P and Q) are disabled by said active condition of such test enablement signal (TEST 2) and thusly not gating of signals, then no signal outputs from either said fourth NAND gate (M) or either of said two fifth NAND gates (P and Q) are interfering with said gating of said test data signal (SS DATA IN) through said first NAND gate (N) and said second NAND gate (O) to which said fourth NAND gate (M) and said two fifth NAND gates (P and Q) are respectively cross-coupled.

7. In an edge-triggered flip-flop embedded into a scan-set shift register functioning as a race-free bistable latch during scan-set testing, said scan-set testing being in response to, and during, an active first test enablement signal--a signal called TEST 1--plus in response to, and during active scan-set clock signals--signals called SS CLK A and SS CLK B--to shift an input scan-set test data signal--a signal called SS DATA IN--to an output scan-set test data signal--a signal called SS DATA OUT, and said scan-set testing necessarily disabling via and during said active condition of said first test enablement signal, both a first input NAND gate--called NAND gate B"--coupled to a second input NAND gate--called NAND gate C"--of said edge-triggered flip-flop, which two NAND gates also receive an active system clock signal--a signal called SYST CLK--in order that, by said disabling via said active condition of said first test enablement signal, both said NAND gates also receiving said active system clock signal will not be satisfied in the development of signals which would conflict with said scan-set testing of said edge-triggered flip-flop, an improved logic apparatus for controlling and interconnecting to those said two input NAND gates which two input NAND gates also receive both said system clock signal--said SYST CLK--and said first test enablement signal--said TEST 1--to the end that said two input NAND gates may be tested compatibly with, at a time after, said scan-set testing of said edge-triggered flip-flop, said apparatus comprising: second test enablement signal source means for supplying, at a time after said scan-set testing, an active second test enablement signal--a signal called TEST 2; first connection means for supplying said active test enablement signal, said signal TEST 2, and in a polarity which will disable, all additional NAND gates of said edge-triggered flip-flop which are cross-coupled with either said first, said second, or both said first and said second of said input NAND gates, said cross-coupling meaning that each said additional NAND gate does receive output signals from, and supply input signals to, one(s) of said first and said second input NAND gates; first supplementary NAND gate means--called NAND gate K--for forming a second test stimuli signal as the inversion of the logical ANDing of said scan-set test data signal--said signal SS DATA IN--and said active second test enablement signal--said signal TEST 2; second connection means for supplying said second test stimuli signal to said first of said input NAND gates--said B" NAND gate--of said edgetriggered flip-flop; merging connection means for observing the output signal of said second of said input NAND gates--said C" NAND gate--of said edge-triggered flip-flop as the output scan-set test data signal--said signal SS DATA OUT; whereby since said first input NAND gate--said B" NAND gate--is coupled to said second input NAND gate--said C" NAND gate--then a complete inverter string consisting of said first supplementary NAND gate means--said NAND gate K--then said first input NAND gate--said NAND gate B"--then said second input NAND gate--said NAND gate C"--exists between said input scan-set test data signal--said signal SS DATA IN--and said output scan-set test data signal--said signal SS DATA OUT; whereby observation of scan-set test data signal flow through said inverter string the correct operation of the NAND gates therein, and particularly said first input NAND gate and said second input NAND gate which are of said edge-triggered flip-flop, may be verified; whereby said testing responsive to said active second test enablement signal after said scan-set testing does test the correct operation of two NAND gates, said first input NAND gate and said second input NAND gate, which are not tested during said scan-set testing.