Baud Rate is a number of sent/received bits per second. In case the UART is used, baud rate depends on: selected mode, oscillator frequency and in some cases on the state of the SMOD bit of the SCON register.
All the necessary formulas are specified in the table:
(*) Baud rate is doubled if PCON.7 (SMOD) is set.
Mode 0: Oscillator / 12 = 11.059 MHz/12 = 921,583 baud
Mode 2: Oscillator / 64 = 11.059 MHz/64 = 172,797 baud
Mode 1 & 3: Baud rate is determined by how frequently timer 1 overflows. The most common method is to configure timer 1 in 8-bit auto reload mode and set a reload value (TH1) that causes Timer 1 to overflow at a frequency appropriate to generate a baud rate.
The following equation is used to determine the TH1 value.
TH1 = 256 - ((Crystal / 384) / Baud) where 384 = 32 * 12
If PCON.7 is set then TH1 = 256 - ((Crystal / 192) / Baud)
To achieve 19200,
TH1 = 256 - ((Crystal / 192) / Baud)
TH1 = 256 - ((11059000 / 192) / 19200)
TH1 = 256 - ((57699) / 19200)
TH1 = 256 - 3 = 253
Timer 1 is usually used as a clock generator as it enables various baud rates to be easily set. The whole procedure is simple and is as follows:
All the necessary formulas are specified in the table:
| SM1 | SM0 | Mode | Mode Description | Baud Rate |
|---|---|---|---|---|
| 0 | 0 | 0 | 8-bit Shift Register | Oscillator / 12 |
| 0 | 1 | 1 | 8-bit UART | Set by Timer 1 (*) |
| 1 | 0 | 2 | 9-bit UART | Oscillator / 64 (*) |
| 1 | 1 | 3 | 9-bit UART | Set by Timer 1 (*) |
Mode 0: Oscillator / 12 = 11.059 MHz/12 = 921,583 baud
Mode 2: Oscillator / 64 = 11.059 MHz/64 = 172,797 baud
Mode 1 & 3: Baud rate is determined by how frequently timer 1 overflows. The most common method is to configure timer 1 in 8-bit auto reload mode and set a reload value (TH1) that causes Timer 1 to overflow at a frequency appropriate to generate a baud rate.
The following equation is used to determine the TH1 value.
TH1 = 256 - ((Crystal / 384) / Baud) where 384 = 32 * 12
If PCON.7 is set then TH1 = 256 - ((Crystal / 192) / Baud)
To achieve 19200,
TH1 = 256 - ((Crystal / 192) / Baud)
TH1 = 256 - ((11059000 / 192) / 19200)
TH1 = 256 - ((57699) / 19200)
TH1 = 256 - 3 = 253
Timer 1 is usually used as a clock generator as it enables various baud rates to be easily set. The whole procedure is simple and is as follows:
- First, enable Timer 1 overflow interrupt.
- Configure Timer T1 to operate in auto-reload mode.
- Depending on needs, select one of the standard values from the table and write it to the TH1 register. That's all.
| Baudrate | TH1 | SMOD Bit | ||||
|---|---|---|---|---|---|---|
| OSC (MHZ) | ||||||
| bps | 11.0592 | 12 | 14.7456 | 16 | 20 | |
| 150 | 40H | 30H | 00H | 0 | ||
| 300 | A0H | 98H | 80H | 75H | 52H | 0 |
| 600 | D0H | CCH | C0H | BBH | A9H | 0 |
| 1200 | E8H | E6H | E0H | DEH | D5H | 0 |
| 2400 | F4H | F3H | F0H | EFH | EAH | 0 |
| 4800 | F3H | EFH | EFH | 1 | ||
| 4800 | FAH | F8H | F5H | 0 | ||
| 9600 | FDH | FCH | 0 | |||
| 9600 | F5H | 1 | ||||
| 19200 | FDH | FCH | 1 | |||
| 38400 | FEH | 1 | ||||
| 76800 | FFH | 1 | ||||
Related topics:
8051 Crystal and Baudrate | 8051 Timer Delay Calculation | 8051 Serial Port | 8051 Serial Port Programming | 8051 Timer Programming | 8051 Timers | 8051 Software Development Tools | 8051 Development Boards and Evaluation Kits | 8051 Features | 8051 History | 8051 Enhanced Version 8052
List of topics: 8051
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