The 2MHz is the frequency of bus accesses from the micro. So if you had a 4MHz system, you'd need a way to slow down accesses for those cycles where the ACIA is selected. There are two main approaches: to change the clock waveform dynamically, or to use the RDY input to stall the micro for one or more cycles.

Or, of course, you can run the whole system at 2MHz, sacrificing some performance for simplicity.

It might be that an alternate choice of peripheral can match the 4MHz speed of the CPU.

I have Intersil CD65C51s that are rated at 2MHz but I have used them at up to 20MHz without any issues. I have used other 65C51s at way past their rated speed as well. I'm not sure what that speed rating even means if it can be exceeded by an order of magnitude. I think if you've got minimal delay in your chip select you can push these chips pretty hard. On the one I tested at 20MHz the chip select from a valid address is only 7ns.

_________________
Bill

That's quite an extreme finding! Of course, 2MHz does mean it will go at least that fast, and evidently it does.

Ok, after considering several alternatives, I think I will stick with MC146818.
Can anyone provide tested schematic showing how to interface that chip witch 6502 based system?

In systems that don’t have a multiplexed address and data bus where I have seen the MC146818 used, they treat the MC146818 as having an address register and a data register. That is, it only takes up two addresses in the memory map. As far as the CPU is concerned, there is no multiplexing.

Mark

Hello,

Please see attached files.
3 examples of RTC-chips on 6502 systems.

BR/
Guus