1. Not old. Vintage. :)

Adding an UART to a 6502

from André Fachat

Adding an UART to a 6502
(C) 1997-2010 André Fachat

I have now stumbled across quite some 6502 computers that use the 6551 ACIA for a serial interface. When I used this chip, I found it rather slow then. Especially as I was doing a Multitasking operating system for the 6502, the interrupt latency became to large to reliably catch each and every character from the device even at 2400 Baud. This was even worse on the 1MHz my C64, into which I had built an ACIA interface in a socket under the SID 6581 (sound chip).

I had already heard of the 'FIFO-serial interface' for PCs, and even replaced some 8250 with 16550A myself. So I decided to replace the ACIA in my C64 with the UART 16550A.

Note: this article has been published in the (now disappeared) "The Computer Journal", Issue #80 Fall 1997, under the title "High Speed Serial for 65xx - How to add a 16550 Uart to a 65xx system".

Bus Differences

The UART chips was designed for a different bus as the 6502, so there are differences in how to handle the chip. When I had a first look at the interface I really wondered why they built the chip this way...

The Reset line is high active - which places the chip in "running mode" at power up, and then only an explicit signal on the Reset line resets the chip. The 65xx series use an active low -RESET line, such that even a simple RC-element with an additional Schmitt-Trigger suffices for a simple computer.

The UART Interrupt line is high active also. You can not wire-or it together with other devices (brain-damaged PC design!) as in 65xx systems for example. And of course, there is no general clock line, but accesses take place during active RD/WR lines, as long as the chip is selected. The UART has two read (RD and -RD) and write (WR and -WR) lines, where each triggers a data transfer (i.e. not (RD and -RD), but (RD or -RD) -- which is IMHO quite peculiar, but I can think of uses for DMA, for example).

These differences in the bus system are, in my opinion, the reason that many people still use the ACIA 6551, and not the much more sophisticated UART 16550A. A more sophisticated chip involves more sophisticated software. In contrary to the simple handling of an ACIA, the UART is indeed more complicated to handle, which also seems a reason for its low usage. In fact there are many caveats in the UART design that make the thing terrible to program. See the Serial FAQ at ftp://ftp.phil.uni-sb.de/pub/people/chris/The_Serial_Port for more information. But then the UART has the already mentioned advantage of the FIFO, which makes it attractive.


When I was rebuilding my ACIA interface to a UART interface, I had to reuse some sockets and chips, because I didn't really want to build everything from scratch (where I didn't have all the parts at home at that time anyway...)

The old Interface card was a daugtherboard to be put between the socket for the C64 SID chip and the SID itself. An additional connector gave three signals: A9 to divide the SID address space in half (although the SID has only 28 registers, they are mirrored in a 1kByte block of I/O space in the C64), -E to enable the ACIA at all and -IRQ to signal the CPU. I soldered A9 and -IRQ to appropriate places on the C64 motherboard, while -E goes to a switch.

With the ACIA and the only use of a dual 2-to-4 decoder 74LS139 I was able to do the whole thing. It even really made the ACIA disappear from the memory map, when it was disabled. This didn't work with the UART, because I needed more circuitry from the decoder to manage the RD/WR handling. Well, one could surely think of something cleaner, but I wanted it quick...

Here's how it works in the C64

All pins from the SID are connected from the C64 SID socket to the SID socket on the daughterboard, except -CS (pin 8). This line is or'ed with the additional A9, to remove the SID from the upper half of its memory window, and then given to the SID. The first half of the '139 then gives the condition that Phi2 is high, -CS is active, and A9 is high. This output is then fed into the -E pin of the second decoder. The enable line (from the external enable switch) switches between the two used and the two not used outputs of the decoder. R/-W as the lower decoder address line then switches between the RD and the WR line. These outputs are or'ed with the inverted Phi2, and fed to the UART.

This arrangements has two purposes: The two decoder stages that have Phi2 in the first decoder give a delay to the beginning of the access. This is needed, as the C64 switches from video chip memory access to CPU memory access with the phases of Phi2. So the address lines need some time to adjust (The VIA 6522, for example, expects the address lines valid at the beginning of Phi2, and doesn't work with the C64 that way. It needs the starting transition of Phi2 to be delayed). The ORing with the inverted Phi2 then stops the access by invalidating the RD/WR lines when Phi2 becomes inactive. This also is a reason for Phi2 being used as CS line.

Another Example

I have also built a Dual UART card with two 16550A for my selfbuilt 6502 computer, and there I used a similar approach. I took a 74LS138, a 3-to-8 decoder. The select line for the I/O area goes to the decoder as enable line (-E1), as well as Phi2 (E3). -E2 is not used and set low. A0 is connected to R/-W of the system, and A1 is another address line. A2 is also set low. The outputs Q0-Q3 are then connected to the two chips.

-Q0 -> UART1 -WR,
-Q1 -> UART1 -RD,
-Q2 -> UART2 -WR,
-Q3 -> UART2 -RD

The Software

Now that you have the UART in your computer, you have to have some software to use it. I have also rewritten the C64 OS to use the UART as serial interface (which I had also done to use the ACIA before). You can find it on my kernal page. But I have already written a generic UART device driver for my selfbuilt 6502 Operating System, OS/A65.

The code shown here is part of that driver, together with a simple C64 binding. It actually follows the suggestions given in the "Serial FAQ" (being mentioned in TCJ#79 to be at ftp://ftp.phil.uni-sb.de/pub/people/chris/The_Serial_Port but not available anymore). I have one problem, though. Because the interrupt generation is somehow buggy in the UART, the FAQ suggests to start the transmitter from outside the IRQ routine. Well, in my OS I don't have any device code outside the IRQ routine (that is called when data is being sent). But then, as the 6502 cannot directly decide where an IRQ came from, the interrupt drivers are (almost) all called when an IRQ occurs, with "higher priority" first. So the serial driver, being the one with the highest priority, is called very often, even if it is not the source of the IRQ itself. But that ensures that the IRQ routine is called and so I can check there, if I have to start transmission manually.

The listed program echos characters it receives from the serial line back to the serial line. It also takes characters typed on the C64 and sends them to the serial line. A part of the screen is used as character buffer, so that you can see something, when the program receives characters

I have written a SLIP (Serial Line Internet Protocol) program for my selfwritten OS, that uses an UART. On my 1 MHz C64 with builtin UART it replies to Internet PING messages without packet loss at 9600 baud. Using the same program with an ACIA, even at 2400 baud, is completely useless due to lost characters.


The 16550A might not be the chip of choice for simple applications, where a high data rate is not necessary. But if you don't want to use the serial line as a terminal line only, but want to do some serious data transfer, better take an UART. Here you see a way to use the UART in 6502 based systems and how to program it.


Here are the schematics and source code for the interface
  • c64ser2.gif is the C64 interface on top of the SID chip (Source: c64ser2.fig
  • c64ser.a65 is a C64 demo program with a UART driver as commented source code.
  • c64ser.o65 is the C64 demo program as loadable binary.
  • c64ser2.a65 is the UART driver from GeckOS/A65.
  • DUART board for the CS/A65 computer.
[ Source: http://www.6502.org/users/andre/icaphw/c64ser.html ]

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