Adding a serial interface - MC6850 ACIA

Adding a Serial Interface

This step will add a serial interface to the SBC and in so doing move a big step closer to having a real computer.  The serial interface will provide for full keyboard entry and an ASCII terminal display.  Very exciting!

My first choice asynchronous communication adaptor would have been a 6551 but at the time I couldn't find one in the UK at a reasonable price so I opted for a related, but more basic MC6850 .  It has no baud generator so the choice of a crystal oscillator circuit was more critical than for a 6551.  Also the 6850 uses only two registers by cunningly differentiating two pairs (four total) by making one pair read only (RX buffer and status register) and the other write only (TX buffer and Control register).  Not as elegant as the 6551 but it works.

Conceptually the circuit is simple enough - the 6850 requires 8 data lines, one register select line (A0), PHI2, RW and a chip select line from the address decoding logic.  On the serial side I used an FTDI serial to USB module - FTDI only because I had one; any should work.  Only three lines to connect - TX, RX and ground.  I have not tried using the IRQ line so far.

Clock Generator

The MC6850 has got three options to divide the clock frequency - 1, 16 and 64.  I chose to use a 1.8432MHz crystal divided by 16 in the MC6850 to yield 115200 baud.

Clock generator for 6850

This is the oscillator circuit:

Note that the choice of NAND gate type is important for this circuit to work correctly.  I used a very old 7400 IC (true retro computing!); an 'LS' series worked too.  HC and HCT types will not work correctly because they are much higher gain and are difficult to bias correctly.  Sometimes old is better 😲

The 6850 allows for the clock to be divided by 1, 16 or 64.  I chose 1.8432MHz because dividing by 16 produces 115200.  Dividing by 64 would yield 28800 but I didn't see any advantage in going slower.

The 6551 has a baud rate generator that allows a number of speeds to be set with one crystal but the 6850 is limited to the crystal divided by one of the three values (set in configuration).

6850 Circuit

Not much too this:

• Connections to data bus
• Clock oscillator
• Address decoding logic to 6850 CS
• RW
• PHI0
• A0 to RS

Watch out RX on the 6850 connects to TX on USB to Serial: TX on 6850 to RX on USB Serial.

6850 Configuration

Only 8 bits to write to the configuration file.  This is what I used:

        %00010101

        Bit 7 clear         - no interrupts

        Bits 6 & 5         - select type of interrupt.  Not used

        Bits 4, 3 & 2     - 8 data bits, 1 stop bit and no parity

        Bits 1 & 0         - divide clock by 16

Assembler Code for Testing

Sending

The steps required are to:

1. Check if the TX buffer is empty by reading the Status register.  Bit 1 signals the TX buffer and is set until the buffer is empty
2. Store a value from the accumulator into the 6850 data register

Polling the status register can be done like this:

 WAIT:    LDA        Status_Register

         AND        #%00000010

         BEQ        WAIT

         STA        Data_Register

Receiving

The steps required are to:

1. Check if the RX buffer is full by reading the Status register.  Bit 0 signals the RX buffer and is set when the buffer is full i.e. a complete character has been received.
2. Load the character into accumulator to clear the RX buffer flag and to allow the next character to be received

Polling the status register can be done like this:

 WAIT:    LDA        Status_Register

         AND        #%00000001

         BEQ        WAIT

         LDA        Data_Register

There is no flow control so the 6502 code must read characters as they arrive - if this is not done an overflow error flag will be raised.

Results

The serial interface transformed what was up to this point mainly a hardware experiment into a useable 6502 SBC!  Having the ability to send and receive from a laptop with full keyboard and screen has been astonishing.  The interface works consistently well and at 115200 is more than fast enough for all my needs.

This short video shows how a terminal on a laptop appears.  Characters are being typed on the laptop keyboard, sent through USB to the 6850 ACIA and received by the 65C02.  The code simply sends the received character back - the characters appearing in the terminal window are coming from the 65C02 via the ACIA:

Where Next?

With the serial interface in place I want to be able to load new code directly into RAM and to execute it.  This requires a basic 'monitor' program in ROM to manage data flow in both directions.  More on these developments in more posts soon!

Another experiment is to compare the 6850 with a 6551 (thanks for the donation Louis!) which includes a baud rate generator so can be configured to run several rates by setting the control register.

Components

Oscillator:

1. 7400 quad NAND gate (or 74LS00)
2. 100nF capacitor
3. 120F capacitor
4. 2 x 330 ohm resistors
5. 1.8432MHz crystal

Other:

1. MC6850 ACIA
2. USB to Serial module