Tynemouth
MINI PET 40/80
19
V1.82
40/80 BUTTON
The 40/80 button is used to switch between 40 column and 80 column video modes. This has the same
debouncing circuit as the NMI button, but without the second stage of inversion. This gives a clean positive
pulse when the 40/80 button is pressed. This clocks a 4013 flip flop, which is wired as a toggle switch. /Q is
connected to D, so that each time a pulse is received, the output goes to the inverse of the state it was
previously in.
The reset pin of the flip flop is fed by the /5V_On signal generated as part of the soft power on (see later). This
will be high when the Mini PET is in standby, so this will keep the flip flop in reset until the Mini PET is switched
on. When it comes out of reset the Q output will always be low, so the Mini PET will always start in the same
mode.
The diode prevents current flow when the output is high, the signal being pulled high by the 10K resistor. This
signal is optionally inverted again by the 74HC86 XOR gate, depending on a DIP switch setting. This allows the
Mini PET to start in 80 column mode if desired.
RESET BUTTON
The reset button is not debounced like the others as this is handled by the CRTC microcontroller, so a pullup to
5V is all that is required. This uses one resistor which is part of the resistor array pulling other CPU pins high.
The microcontroller generates a Ready signal once it has booted. This is initially held low by a 10K resistor (part
of one of the IEEE-488 resistor arrays) as the microcontroller does not drive any outputs during its reset
period. It is also held low for a further 100mS to generate a reset pulse long enough for the 6502 and IO chips.
The pulse feeds an open collector 74LS07 gate which will pull the 6502 reset line low, but cannot drive it high.
It is pulled high by a 10K resistor (also part of one of the resistor arrays).
The Ready signal also drives an LED which will illuminate after the reset pulse, showing the system is ready.