Hi, experts.

I need to test an R6520 from a microcontroller. The data sheets for the R6520 and M6820/6821 indicate 100 kHz as the minimum value for frequency (maximum period of 10 usec).
Some information from Commodore says that the MC6821 is a completely static device. On the other hand I saw a VHDL code that emulated the MC6821, without any minimum frequency restriction.
Will the R6520 or MC6821 work below 100 kHz?

Thanks in advance.

I've had a MOS 6502 run well at 50kHz, but I can't guarantee any other 6500 series chip will run at that speed. Just give it a try.

Edit: Oops, so if it fails you won't know if it's because it's running too slow or the chip is bad. Why do you need to run it it so low?

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Bill

Maybe have a look at the WDC65C21. This chip is still being produced and IIRC fully static.
Does it need to be the old M6821 or R6520?

Edit: oops you want to test an original chip. What Microcontroller are you using to test it? I believe you can run Arduinos fast enough to run the chip above 100kHz. 16MHz/100kHz is 160 cycles... Maybe not in C though.

Or you can test the PIA in a known working computer like the Commodore VIC20, the PET, or even the VC1541.

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Author of the GeckOS multitasking operating system, the usb65 stack, designer of the Micro-PET and many more 6502 content: http://6502.org/users/andre/

Also, if what the data sheet means by "static" is that the E line does not have to keep pulsing when not doing an operation on the chip, then it seems like a circuit could be rigged up to run through two clock cycles when triggered. A clocked latch to catch read values from the bus at the right part of the process and then the MCU doesn't have to be running at an effective ~50kHz (for the falling select line transition in the middle of Phi1 before the rising E clock).

Don't know about the Motorola MC6821.

But MOS 6520 appears to have only fully static latches insdie, so it is supposed to run at very low frequencies.
For a detailed MOS 6520 chip dissection, go here.

Not strictly related to this but when I was building my 6507 Project I found I could move the wire on the breadboard from a 1Mhz oscillator to a 2Mhz oscillator (and back again) and other than it just stopping when the wire was unplugged for a couple of seconds the thing just kept on running as if nothing had happened.

These were Rockwell 6507s if that makes a difference...

I never tried it for more than a few seconds but it was mildly amusing at the time.

-Gordon

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--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/