I'm currently designing a 7400 series circuit to drive LEDs from a set of shift registers (74595). Each shift register's tri-state input is wired to the output of a 2:4 decoder (74139), and the decoder's input is driven by a 4 bit counter (74191). The counter has a 200Hz clock input.
In steady state, this circuit runs fine. My concern is during the transition. According to the 74LS139 and 74HC595 datasheets, the decoder will transition from one output state to another faster than the tri-state output of the shift registers. This means that there is a brief period of time (about 10ns) where both shift registers will drive their outputs before they complete their transition.
My question is, will this brief transitional period damage and eventually destroy the shift registers? I've attached a simplified schematic illustrating my question.
Do note that this is a simplified schematic. The real circuit has way more shift registers. As such, multiplexing each shift register is prohibitive (I would need a 96:32 multiplexer, a.k.a. lots and lots of 7400 series parts).
Best Answer
I don't think it's a problem provided you aren't switching between the shift registers at silly speeds (MHz instead of kHz). I base this on:
the 10ns overlap is between typical \$t(en)=15ns\$ and \$t(dis)=23ns\$ on the 595 datasheet. So it's less than half the turn-off times. I doubt that the drivers will both be fully on during this overlap.
Drive strength (to the TI datasheet) is guaranteed 6ma, quite low. Worst case drive current will be several times this figure - maybe 25ma at 5V, 0.125W * 8 (no. of outputs). A permanent short circuit may eventually destroy the drivers through overheating, but:
The duty cycle and thus the excess heating is quite low: 10ns per (insert your switching period) - multiply the power above by this (rather small) number.
Finally, if you are still worried about this, you could use OEn on the 74139 decoder to disable both 595s during such transitions.