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docs: add UART documentation

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slederer 2024-10-04 00:29:40 +02:00
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doc/mem.md
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@ -31,6 +31,6 @@ Currently, only I/O slots 0-3 are being used.
|I/O slot| Address | Controller |
|--------|---------|------------|
| 0 | $800 | UART |
| 1 | $880 | VGA |
| 2 | $900 | SPI-SD |
| 1 | $880 | SPI-SD |
| 2 | $900 | VGA |
| 3 | $980 | IRQC |

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doc/uart.md Normal file
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@ -0,0 +1,44 @@
# UART
The UART a single register at address $900.
It uses a fixed serial configuration of 115200 bps, 8 data bits, 1 stop bit, no parity.
## Reading the UART register
|_bit_ |31|30|29|28|27|26|25|24|23|22|21|20|19|18|17|16|
|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_Value_|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_bit_ |15|14|13|12|11|10|09|08|07|06|05|04|03|02|01|00|
|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_Value_|- |- |- |- |- |- |A |B |d |d |d |d |d |d | d | d |
|Bitfields|Description|
|---------|-----------|
| _A_ | data available, at least one byte has been received
| _B_ | TX busy, a byte is being transmitted and no data can be written
| _d_ | 8 data bits
## Writing the UART register
|_bit_ |31|30|29|28|27|26|25|24|23|22|21|20|19|18|17|16|
|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_Value_|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_bit_ |15|14|13|12|11|10|09|08|07|06|05|04|03|02|01|00|
|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
|_Value_|- |- |- |- |- |T |C |- |d |d |d |d |d |d | d | d |
|Bitfields|Description|
|---------|-----------|
| _T_ | transmit enable, writes a data byte
| _C_ | clear receiver, acknowledges a received byte
| _d_ | 8 data bits
## Notes
A 16 byte FIFO is used when receiving data.
When reading data, each byte needs to be acknowledged by writing the _C_ flag to the UART register.
When the FIFO is empty, the _A_ flag in the UART register will be 0.