UART Board
Overview
This is the second board I made in 2017. I've made some changes to it in 2020 when I started to add more boards to the system. Before, the address decoding was done with a few different 74LS logic chips. I replaced it with a single 74LS128 3 to 8 demux. I was also having issues with the 16c550 not resetting correctly, so I tied the reset signal into the computers reset circuit. It should have been like this from the start. I needed to invert the signal, so I used a 74LS00 NAND since this was already on the board from the old address decoding.
The crystal for the UART is 1.843200MHZ. We write 12 to the divisor latch. Then, it is divided internally by 16. This gives us 9600 baud. You can see this in uart.s in the SetupUART routine.
I still have issues with this chip not resetting sometimes, so more investigation is needed.
