A4.8051 Communication Protocols:UART, RS232
Contents
UART Serial communication using 8051
We have covered the basics of timers in the previous tutorial, with that we can see how can a microcontroller communicate with a computer serially.
There are two ways to connect a microcontroller to a computer.
- Using RS232 protocol to the legacy serial port.
- Using a USB to serial convertor.
From microcontroller programming point of view, there is fundamentally no difference.
8051 UART Registers
SBUFF: Serial buffer register
| SBUFF | |||||||
|---|---|---|---|---|---|---|---|
| D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
SCON: Serial Control Register
| SCON | |||||||
|---|---|---|---|---|---|---|---|
| D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
| SM0 | SM1 | SM2 | REN | TB8 | RB8 | TI | RI |
*SM0 - Serial port mode bit 0 is used for serial port mode selection.
- SM1 - Serial port mode bit 1.
| SM0 | SM1 | Operation | Description | Baud Rate Source |
|---|---|---|---|---|
| 0 | 0 | Mode 0 | 8bit ShiftReg | 1/12 the quartz frequency |
| 0 | 1 | Mode 1 | 8 bit UART | Determined by the timer 1 |
| 1 | 0 | Mode 2 | 9 bit UART | 1/32 the quartz frequency |
| 1 | 1 | Mode 0 | 9 bit UART | Determined by the timer 1 |
*SM2 - Serial port mode 2 bit, also known as multiprocessor communication enable bit. When set, it enables multiprocessor communication in mode 2 and 3, and eventually mode 1. It should be cleared in mode 0.
- REN - Reception Enable bit enables serial reception when set. When cleared, serial reception is disabled.
- TB8 - Transmitter bit 8. Since all registers are 8-bit wide, this bit solves the problem of transmiting the 9th bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th bit.
- RB8 - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is a logic 1.
- TI - Transmit Interrupt flag is automatically set at the moment the last bit of one byte is sent. It's a signal to the processor that the line is available for a new byte transmite. It must be cleared from within the software.
- RI - Receive Interrupt flag is automatically set upon one byte receive. It signals that byte is received and should be read quickly prior to being replaced by a new data. This bit is also cleared from within the software.