The 82C52 UART

The 82C52 is a single channel UART. Here is my schematic:

It is quite adaptable to different clock frequencies. Using the M80C85A-2 clock of 5 MHz (i.e the maximum recommended clock speed), here are the available baud rates:

Note: A 9.8304 MHz crystal would make these baud rates exact.

Going through my crystal junk box I find an 11.0592 MHz crystal. I could use this directly on the 82C52 and get a large range of exact bauds:

Anyway I will stay with the 10MHz crystal for the M80C85A-2.

Programming the 82C52

I have fully decoded an address of 0x0040 (i.e. 0x40-0x43) which in reserved for the CBIOS in the zero page. I guess it is not often used as it takes some effort to decode.

Here are the initialisation codes for 9600 8N1:

• 0x40   Read/write serial data
• 0x41    Set 8N1                                                 0x3C
• 0x42   Use INTR with DTR/CTS                      0x66
• 0x43   Set baud 9600 using a 5MHz clock   0x2A

The decoding scheme can be used for up to to 3 other devices using the CBIOS reserved area.

AlanX