CPU Reset Lines

What does a CPU reset line do? It forces the CPU to restart in a known state. This state is where the CPU registers are properly initialized and the control signals are set to known levels. The CPU will access the area of memory where it gets its first instructions and it starts executing them.

Look at the signal. If it is labled as RESET* (or with a bar over the text) it is a logic low active signal. This means that to reset the CPU you pull the line down to a logic low state. (no bar or * then it's active high)

When the board is powered on the reset* line will be held at a logic low level for at least 1/10th of a second before going to logic high. Some CPUs are very sensitive to this. Motorola 68000 CPUs aren't and can be reset with a short 50ms pulse. A Z80 CPU requires a longer reset signal to properly initialize.

If the reset signal isn't long enough or is stuck at the wrong level then it's time to investigate.

If it's stuck active or oddly pulsing then look for a short on the reset line. It could be from a defective IC, a bent pin on a chip that is shorting to the reset line, or a shorted capacitor in the RC network. Time to trace the signal all throughout the board. Take your time as this signal can go to chips all over.

If it's stuck inactive and never resets the CPU then look for a broken trace or bad reset circuit. Some are done by a chip such as MB3771 or MB3773 but most are done simply by using an RC network. Follow the traces on the board back to a resistor and capacitor connected to power and ground. The resistor limits the charging current to the capacitor. The capacitor holds the reset line low until it charges. If the capacitor has a leg broken, is leaky, or is open internally then the reset line will never be held low enough to initialize the IC.

In the case of a Z80 IC not being reset long enough the symptoms will be NO signals on the address or data lines when examining them with a logic probe. Fix the reset line then the address and data lines will come to life.

Some boards have a circuit known as a "watchdog circuit." This circuit has a specific job: watch the CPU for activity. If the activity isn't seen then reset it. When problems occur with a board where you see the video output or you hear the audio output have a specific reocurring 'tick' check the CPU's reset line. If the reset line periodically changes logic state then you have a "stuck in watchdog" problem.

When this occurs check the CPU, boot ROM (or ROMs if using a 16bit CPU), and work RAM. Memory decode circuits, RAM/ROM enable lines, chip sockets, or cut/gouged traces can all cause this problem. It's just a matter of checking these things one by one. Do the easy things first. Test any RAM/ROM/CPU chip that's socketed. Next check for continuity on the address/data lines then check logic signals on the RAMs/ROMs. Continue down the line until the problem is found.

Enjoy!

RJ