Motorola 6850 UART

This 6850 UART serial chip was responsible for MIDI in many vintage synthesizer. The Prophet 600 used this chip as one of the first MIDI synthesizers produced.

Baud rate generation

In many UART chips, such as the intel 8251 we need to have an external clock to produce the required baud rate, with this chip it’s really easy to do.
We can internally divide by 16 a 500KHz signal into the required 31,250 baud we need for MIDI. So if our synth has a master clock for the CPU. We can use a 4013 or similar flip flop to divide this down to the 500KHz we need.
The BBC Micro interface used this on it’s 1MHz bus and just divided the clock by 2.

Of course we could use the programmable interval timer (82C54) to generate any baud rate we want.

Control pins

One thing I had to address was the R/W pin, instead of a RD and WR that’s traditional on an 80s computer we have one pin. I connected this to the write (WR) line which meant that the chip was always in read mode when the chip select went low. I used a NAND gate with the RD and WR signals and the output of this to the E (enable) pin.

For some reason they put lots of chip select and enable pins but not individual read and write lines?

To send data you just setup the chip with the control words, then send bytes to it and it will send. To receive you can do one of 2 things.. program the IRQ interrupt pin to trigger the microprocessor/controller or read the status register of the pin for data flag then read the bytes in the buffer.

Nowadays most MCUs have built in UARTS into AVRs/PICs, so why use old chips. My answer is simple, it’s just more fun and will teach you so much more about digital electronics.

The theory is the same inside a modern microcontroller we have data, address, control buses and UARTS just all inside one chip rather than having to use external chips for different jobs.

The 6850 UART serial chip – A great little chip for retro computing