gfoot wrote:
Isn't it better to just use a different part? Does the 6551 have some benefit over the other parts that makes up for the bug?
There are multiple reasons, for my videos series specifically.
For one, I want to build a computer that any viewer can reproduce on breadboards. This means using in-production or easily swappable parts. It also means using DIP ICs only. Now the only DIP UARTs you can get at Mouser are the W65C51N, the MAX3100, and some Z80 family chip. That is not a lot of options to work with, unfortunately.
I'm also making educational videos, so I need something that can easily interface with the CPU, so the focus can be on what the UART does and not on the interface. That makes the MAX3100 with its SPI bus a more advanced option, maybe for later revisions.
Showing the bug and how to work around it also is good educational content.
Of course, if you can get an R65C51, that's probably a better version, but not everyone wants to deal with second-hand chips. It is also specced for a lower bus frequency, which means there could be the additional issue of wait states to deal with. I have plans to later upgrade to a TL16C550CN. The DIP version
is still findable new, but not in regular distributors, however. It will be a nice upgrade.
With the VIA's PB6 input trick for a timed tx interrupt, I think the W65C51N will honestly work well enough. It is a fine option for anyone making a breadboard computer as long as you know this trick, in my opinion.
floobydust wrote:
Well, you do have an engineering sample W65C51 UART. Several years ago Bill Mensch sent me 6 engineering samples before the current part was put into production. The one you have is the same as two of mine... and yes, I have one working, but the second one has some odd issues. I've run the chip with a can oscillator of 3.6864 MHz for a quicker baud rate and with a CPU clock up to 10MHz without any issues. I'm also using interrupt-driven transmit and receive. I wouldn't count on finding more of these, and even if you do, there's the possibility that all will not function the same.
Attachment:
W65C51S-PCB.jpg
I would strongly suggest going to a different chip if you plan on making more than a single system. The NXP (D)UARTs are excellent and have more functions and higher baud rates available.
Cool to see I'm not the only one with these!
Yeah I agree I'm not sure how much I should trust these devices, there is at least one known errata and there could be more given that they haven't really been tested by a lot of people. I will definitely use them in personal projects but not on the videos.
See above for why NXP UARTs are not an option for me at the moment
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