AVR as UART/ACIA IC

[BigDumbDinosaur]

cbscpe wrote:

But if you can live without modem control signals (and honestly who does need them nowadays)...

Anyone who runs a FAX server will have at least one modem and definitely will need modem control signals. No matter what, you do need CTS and RTS if you plan to have reliable, high speed serial communication.

[cbscpe]

Yes but then you can use a normal 6551 without the additional CPDL.

[brain]

But, then you are limited to the slower bps rates. a 6551 + CPLD gives you best of both worlds.

[brain]

cbscpe wrote:

At mouser the small CPLD ATF1504 and a W65C51 cost together about 12 swiss franks and the larger CPLD ATF1508 costs 8 swiss frank. I know that the receiver and the transmitter with buffering fits in a ATF1508 (i have done this and the design requires about 70 flipflops). Whether all the other features still fit in the ATF1508 I need to check. But if you can live without modem control signals (and honestly who does need them nowadays) it should not be a problem and as the ACIA only requires very few pins you can use the rest of the pins to build your address decoder. a address decoder typically requires alot of pins but very little CPLD resources. In other words they complement each other perfectly.

If we can stuff the 6551 the registers into a 128 macrocell CPLD, I'm game. The 6551 requires at least 48 registers to operate, as far as I can tell (8 for XMIT + 8 for buffer, 8 for RECV + 8 for buffer, 8 for COMMAND + 8 for CONTROL). I suspect the status flags take some as well.

But, if I could get it into a 128 or even a 256 macrocell CPLD, I'd be happy to try.

Jim

[cbscpe]

Hi,

here is the receiver and transmitter section written in WinCUPL for another project that is ongoing but is not 6551 related.

UART1508P.TXT

Basic UART Code

(17.09 KiB) Downloaded 155 times

Whether all the missing features still fit one has to check. If one restricts himself to the features really needed then building a 6551 replacement should be possible. It really depends on the application what really is required. At least the annoying TX IRQ bug will not be included

Cheers

Peter