I Think I Just Killed My TI (was Reset Button)

[matthew180]

The 9904 (clock chip and system reset generator) is dead, probably due to the short between CPU pins 5 and 6. Even though pins 5 and 6 are inputs to the CPU, they are driven by the reset output of the 9904, and grounding that output for an extended period (which is a matter of seconds to a chip) appears to have shorted out the 9904's reset output driver stage. The system's 5V line was being drawn down (I could watch it drop with my meter) to about 4.1V and the 9904 was getting hot enough to burn you to the touch (which I almost did). Removing the 9904 stabilized the 5V.

 

Based on this I would have suggest that wiring a reset button between ground and CPU pin-6 is risky at best, and you are risking blowing your 9904 every time you hit the reset. For those who have done a reset button this way without damage, it is probably because the reset button is only held for a brief moment. I would suggest a better reset would be to short the RC circuit via a switch and small resistor on the *input* side of the 9904. Same switch to ground with optional resistor, but *much* safer.

 

So, does anyone have a spare 9904 laying around? This *could be* a cheap fix if I can source a 9904.

[+Ksarul]

Let me check, I think I have a tube of TIM9904s around here somewhere, Matthew.