willie68 wrote:
Just a short comment the R65C51 (from china) are driving me crazy. I have 4 of them, bought together, but every chip from the outside is different. 3 PlasticDip one ceramic, 3 with the Rockwell logo one without…
And in my PCB every chip behaves different. The ceramic chip doesn’t the oscillator, one give always an interrupt, bit 7 even if thee s no other bit set. One uses always 5 Bit serial, no matter what I put in the control register, one alway output %00000000, the last has a different baud rate.
I’ll give up to use the 6551 for my SBC, have ordered a pair of max3100…
Well, just to add a bit more information....
- The current WDC W65C51 has a defect, where the transmit interrupt flag is stuck on, so that's a problem.
- Some 6551 versions require a 1 megohm resistor in parallel with the crystal to oscillate, some don't (the new WDC part does).
- Rarely will you get a 65(C)51 to run much higher than the rated speed (note: I only run mine with interrupt-driven receive and transmit).
I have quite a collection of them... early Rockwell 6551, Synertek 6551, Rockwell 65C51, about 4 different versions of W65C51 from WDC directly, some of the latest WDC released chips and likely one or two others. In general, I've managed to get pretty much all of them working, but there are a few problem ones in the mix.
Some years ago, I purchased some small quantity of Rockwell CMOS chips from utsource.net. This included 10- R65C51P4, 10-R65C22P2 and 10-R65C02P4. All CMOS 4MHz rated parts. So far so good... they work, albeit the R65C22P4 parts have very fragile leads... one broke off on a chip. In any case, as you already have the PCB setup for the 6551, You'll probably have good luck purchasing a few R65C51P4 from utsource.net, as they still show stock. My Micromon code (github) will drive a 6551 as a console with interrupt-driven and buffered receive and transmit, which will allow you to sort out which chips work and which ones don't.
https://www.utsource.net/itm/p/1524790.html