Tynemouth
MINI PET 40/80
18
V1.82
KEYBOARD MATRIX
The keyboard matrix matches that of the PET Chiclet keyboard, with 10 rows and 8 columns
There are four additional buttons on the Mini PET 4080 keyboard which are not part of the normal keyboard
matrix.
NMI BUTTON
The NMI button provides a “None Maskable Interrupt” to the 6502 CPU. This is debounced by the circuit
above. The Schmitt Trigger inverters used are also used for the soft power on circuit, so run at 9V. In normal
operation, the 100nF capacitor will charge up to 9V. The output of the second inverter will be 9V. The diode
stops this being passed on and the NMI pin is pulled to 5V via one of the pullup resistors in the resistor array
by the CPU.
When the NMI button is pressed, the 100nF capacitor is discharged via the 100K resistor. Once the voltage
across this goes past the lower threshold, the first Schmitt trigger output goes high, and the second Schmitt
trigger inverts this and current flows to ground via D3.
When the button is released, the 100nF capacitor starts charging again via the 10K and 100K. The output will
not immediately change because it is a Schmitt trigger; it will not change state until it has charged past a
higher threshold. This should mean a single clean negative pulse is generated each time the button is pressed.