Clock Speeds?

Discussions about Sinclair ZX80 and ZX81 Hardware
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ZeddyRepair
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Joined: Sun Mar 24, 2019 1:26 pm

Clock Speeds?

Post by ZeddyRepair »

I know the Z80 CPU clock speed is set from the ULA Pin 14 @ 3.25Mhz but does anyone know what the Ram Clock Speed & Rom Clock Speed should be from pins 12 & 13?
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1024MAK
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Re: Clock Speeds?

Post by 1024MAK »

Hello and welcome to our forum.

The frequency of the signals on the ULA pins 12 and 13 depends on the ratio of RAM to ROM accesses. They don’t operate at specific defined frequencies.

Pin 12 is the /RAMCS signal from the ULA to the on board SRAM.
Pin 13 is the /ROMCS signal from the ULA to the on board ROM.

The ULA takes some of the Z80 CPU control signals including some address lines and uses these signals to produce these two signals. They then control the relevant memory chips.

See the pin outs thread here.
Schematic
Updated Schematic (some errors corrected), but with French video circuit

When BASIC is running, most of the memory access will be ROM, with some reads and writes to the RAM. With a machine code program running, most accesses will be to the RAM.

If either signal stays low or stays high, this indicates a fault.

Mark
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ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: Clock Speeds?

Post by ZeddyRepair »

Thanks for the help!

This is good news as i wasnt getting a constant pulse from these outputs with the Oscilloscope so thought maybe the ULA was faulty.

Ill probably start a new thread with the main reason I asked this question......
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