I'm not sure what they mean by the speed rating on a 65C51. I have lots of these from various manufacturers with speed ratings of 1mHz, 2mHz and 4mHz. I have a 1mHz unit working happily with a 6502 running at 4mHz and a 2mHz one running, as I write this, quite happily with a W65C02 running at 16mHz. Does everyone else see this with these chips?

_________________
Bill

Tell us your secret, Bill! How do you do it?

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

I plugged it into the socket and hoped for the best!!??

The thought of having to write CPU clock speed/baud-rate dependent code to use W65C51s forced my hand, my shaking hand - but it turned out good.

Still, what do the specifications really mean? Do we trust them?

_________________
Bill

Garth and Ed have some pertinent comments here. All the points are relevant, but this remark below was an eye-opener for me -- something I wouldn't have thought of.

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

I have a decent collection of both NMOS and CMOS 6551/65C51 chips. Vendors include Synertek, Rockwell, Intersil and WDC. Here's what I've found on speeds:

Rockwell NMOS parts: Good for rated speed and rarely more (1 and 2 MHz versions)
Rockwell CMOS parts: Good for rated speed and rarely more (4 MHz versions, some run @ 5MHz)

Syntertek NMOS part: I have one which will will run rock-solid at 6MHz

Intersil CMOS part: I have one rated at 4MHz. It runs at that speed but not any faster. It also has corrupt data from time to time.

WDC CMOS parts: I have three older LOT numbers which they sent me years ago.... all require a can oscillator to work, one or two run well up to 10MHz but occasionally have hiccups. The latest LOT number (with the XMIT bug) also runs at rated speeds, but it's a moot point (the XMIT bug).

So, all I can say is you have one special chip there... based on my findings, yours is a nice "fluke" to have. Do you have any additional chips you can test?

_________________
Regards, KM
https://github.com/floobydust

That really fast one is an Intersil CDP65C51AE2 with a date/batch code of P0131ACWX and I have more than 5 full tubes of them (75+).

I'll be testing some of them...

_________________
Bill