Question

In 8051, for serial data communication, what does mode 1 means?
1. Multiprocessor variable mode
2. Standard UART mode
3. Multiprocessor fixed mode
4. Shift register mode

Answer 1

Correct Answer - Option 2 : Standard UART mode

Serial Data Transmission Modes:

Mode 0:

• In this mode, the serial port works like a shift register and the data transmission works synchronously with a clock frequency of fosc /12.
• Serial data is received and transmitted through RXD. 8 bits are transmitted/ received at a time.
• Pin TXD outputs the shift clock pulses of frequency fosc /12, which is connected to the external circuitry for synchronization.
• The shift frequency or baud rate is always 1/12 of the oscillator frequency.

 

Mode 1:

• In this mode, the serial port functions as a standard Universal Asynchronous Receiver Transmitter (UART) mode.
• 10 bits are transmitted through TXD or received through RXD.
• The 10 bits consist of one start bit (which is usually '0'), 8 data bits (LSB is sent first/received first), and a stop bit (which is usually '1').
• Once received, the stop bit goes into RB8 in the special function register SCON. The baud rate is variable.

 

Mode 2:

• In this mode 11 bits are transmitted through TXD or received through RXD.
• The various bits are: a start bit (usually '0'), 8 data bits (LSB first), a programmable 9 th (TB8 or RB8) bit and a stop bit (usually '1').
• While transmitting, the 9 th data bit (TB8 in SCON) can be assigned the value '0' or '1'.
• The baud rate is programmable to either 1/32 or 1/64 of the oscillator frequency.

 

Mode 3:

• In this mode 11 bits are transmitted through TXD or received through RXD.
• The various bits are: a start bit (usually '0'), 8 data bits (LSB first), a programmable 9 th bit and a stop bit (usually '1').
• It is same as mode-2, except the fact that the baud rate in mode-3 is variable (i.e., just as in mode-1).