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Timer Retrofit For Z80 SBCs

Quite a few simple Z80 SBC systems don't have a timer interrupt or clock of any kind. CP/M 2.2 after all didn't need one.

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The board itself is a simple clock generator using a 74HC4060 14 stage ripple counter. 2.45MHz goes in one end, 150Hz comes out the bottom. Along the way it generates a bunch of useful other clocks for serial ports.
Because almost all the SBC systems without timers seem to use Z80SIO or 16x50A UARTs there is no bus and interrupt line logic. Instead the 150Hz from the board can be fed into one of the modem control inputs and the UART itself provides both interrupt masking and control.

timertick.kicad_pcb

x-kicad-pcb - 68.81 kB - 11/09/2019 at 14:00

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timertick.sch

x-kicad-schematic - 4.82 kB - 11/09/2019 at 14:00

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  • 1 × 74HCT4060 Logic ICs / Counters
  • 1 × 0.1µF capacitor
  • 1 × 8pin can oscillator to suit your needs
  • 1 × Misc Headers
  • 1 × Double sided sticky tab to stick onto sbc

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freefuel wrote 07/04/2023 at 00:59 • point

Now that I've found this I'm reminded of a watchdog timer I have been thinking of, it was something along the lines of 2 to 4 32-bit programable counters running at 1x to some Nx divided down rate, when the timer trips it resets to it's programmed setting while outputting one of the z80 interrupts so it can be used in any of the three modes. 

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