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Download a datasheet and see.what will be the pin configuration for 7400
That would be true only if the QC output were used directly as the NAND input and is precisely why caps C3 and C4 were introduced in the circuit in post #17, to convert an otherwise prolonged signal to a brief pulse !The Nor gate wont work because the output of the normal clock circuit bit is sometimes high for relatively long periods of time which prevents clocking with the push button. The Nand gate will fail for the same reason only this time when the normal clock circuit bit is low, not high.
Not so. For the CD4011, pins 1,2,5,6,8,9,12,13 are data inputs; pins 3,4,10,11 are outputs.rest of the connections are same.
For the circuit shown:one more thing can you help me in understanding the logic behind using nand gate
C3 or C4 can drive the gate input beyond the +ve rail voltage, but as it's only 10nF it doesn't have enough stored energy to harm the gate input protection diodes (at least, so far I've been lucky when using 10n in similar situations !). If the OP is really concerned then a limiting series resistor between the cap and the gate input, and external diodes from the input to the power rails could be provided. That's starting to look less elegant than your XOR proposal.There's also a small concern about the input when the normal clocking pulse changes state...does it overdrive the input? We'd have to check that for both high-to-low and for low-to-high transitions.
Slow rising cap charge is no problem as the 7490 is negative-edge triggered. Cap C1 or C2 is shorted by button actuation, so discharges very rapidly (say 10 Ohm x 100nF = 1uS) to give a good negative edge, with a re-charge time constant of 100nF * 100k = 10mS for debounce.There's a chance that with the slow rising cap charge or discharge we might see multiply clocks getting through
Slow rising cap charge is no problem as the 7490 is negative-edge triggered. Cap C1 or C2 is shorted by button actuation, so discharges very rapidly (say 10 Ohm x 100nF = 1uS) to give a good negative edge, with a re-charge time constant of 100nF * 100k = 10mS for debounce.
Cap C3 or C4 discharge also gives a sharp negative edge (as fast as the 7490 switches, say 100nS), with a re-charge time constant of 10nF * 10k = 100uS.