8051 Serial Port

• 8051 has integrated Universal Asynchronous Receiver Transmitter (UART) or Serial Port
• Serial port mode of operation and baud rate can be configured

Registers:

SCON
Bit	Name	Function
7	SM0	Serial port mode bit 0 (see below)
6	SM1	Serial port mode bit 1 (see below)
5	SM2	Mutliprocessor Communications Enable. When set the "RI" flag will only be triggered if the 9th bit received was a "1"
4	REN	Receiver Enable. This bit must be set in order to receive characters
3	TB8	Transmit bit 8. The 9th bit to transmit in mode 2 and 3
2	RB8	Receive bit 8. The 9th bit received in mode 2 and 3
1	TI	Transmit Flag. Set when a byte has been completely transmitted
0	RI	Receive Flag. Set when a byte has been completely received

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 (*)

(*) Baud rate is doubled if PCON.7 (SMOD) is set.

Serial Port Initialization:

• Set mode in SCON register
• Set 9^th bit for mode 2 and 3

Set Baud Rate:

• 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)
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

Transmit Data:

• To send a byte to the serial port one must simply write the value to the SBUF (99h) SFR
• Wait until TI bit is set

Receive Data:

• Wait for the 8051 to set the RI flag
• Read the value stored in the SBUF (99h) SFR

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Related topics:
8051 Register Bank and Stack   |   8051 Flags Bits   |   8051 Program Flow   |   8051 Timers   |   8051 Interrupts

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List of topics: 8051

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