Hi,
I was hoping someone in this ng might be familiar with the older 80C48 microcontorller. It's driving a unit which is controlled via serial data over a wire pair. The RESET pin is clocking regularly (about 1 hz) and so are all pins on Port 2. Port 1 is stable.
I'm wondering about the RESET itself. My understanding is that it should be clocked once on power up or power down conditions. I think it powers up on a logic high. At least, on my setup, it is driven directly off the output of a NOR gate. One input of that NOR gate is driven off the collector of an NPN transistor. The gate is steady at
+5 volts with power on. The other gate input is more complex.There is a 0.022 capacitor feeding from the NOR gate output back to the other input via a 1 meg resistor. I assume this is an RC circuit to give a monetary pulse to enable the gate. This input is also fed by another NOR gate output, and it's input is fed from the same capacitor via a 100K resistor. The other input for the second NOR is fed via another RC circuit from one pin of Port 2 and two NOR's wired as inverters.
I'm thinking I have a race condition happening. Instead of the RESET doing it's thing during power on, at least one of the input gates must have an unstable condition. So the controller keeps toggling.
Is this a reasonable assumption, or could it be that the microcontroller needs it's normal serial data from the external machine?