Understanding 8051 Serial Communication Modes

The 8051 microcontroller has a built-in serial communication port that can operate in four different modes. These modes offer varying features in terms of data format, baud rate generation, and functionality like multiprocessor communication. Let's analyze the features of each mode to correctly match them with the descriptions provided in List II.

Analysis of 8051 Communication Modes and Features

List I presents the four communication modes of the 8051, and List II provides their features. We need to match each mode to its correct description.

Matching Each 8051 Communication Mode

• Mode 0: This is the simplest mode, often called the Shift Register Mode. It uses an 8-bit shift register for transmitting and receiving data. The serial clock is the Timer 1 overflow pulse divided by 12 (or system clock frequency divided by 12). This mode has a fixed baud rate based on the crystal frequency.
  • Feature (I) describes a high-speed, 8-bit shift register mode with a baud rate of \( f/12 \) (where \( f \) is the system clock frequency). This perfectly matches Mode 0. Thus, (A) matches (I).
• Mode 1: This is the standard 8-bit UART mode. It transmits and receives 10 bits: a start bit (0), 8 data bits (LSB first), and a stop bit (1). The baud rate is variable and is typically generated by Timer 1 configured as a baud rate generator. The baud rate is determined by the overflow rate of Timer 1 and the SMOD bit in the PCON register.
  • Feature (II) describes a standard 8-bit UART with a variable baud rate using Timer 1 overflows. This accurately describes Mode 1. Thus, (B) matches (II).
• Mode 2: This is a 9-bit UART mode primarily used for multiprocessor communication. It transmits and receives 11 bits: a start bit (0), 9 data bits (8 data bits and an adjustable 9th bit, TB8/RB8), and a stop bit (1). The baud rate in this mode is fixed, determined by the system clock frequency divided by either 32 or 64, controlled by the SMOD bit in the PCON register.
  • Feature (IV) describes a Multiprocessor 9-bit UART with two possible baud rates of \( f/32 \) and \( f/64 \). This accurately describes Mode 2. Thus, (C) matches (IV).
• Mode 3: This is also a 9-bit UART mode used for multiprocessor communication, similar to Mode 2. It transmits and receives 11 bits (start, 9 data, stop). The key difference from Mode 2 is that its baud rate is variable, generated by Timer 1 overflows, just like in Mode 1.
  • Feature (III) describes a Multiprocessor 9-bit UART with a variable baud rate using Timer 1 overflows. This accurately describes Mode 3. Thus, (D) matches (III).

Summary of Matches

• (A) Mode 0 matches (I) High speed; 8-bit shift register; one baud rate of \( f/12 \).
• (B) Mode 1 matches (II) Standard 8-bit UART; variable baud rate using timer 1 overflows.
• (C) Mode 2 matches (IV) Multiprocessor 9-bit UART; two baud rates of \( f/32 \) and \( f/64 \).
• (D) Mode 3 matches (III) Multiprocessor 9-bit UART: variable baud rate using timer 1 overflows.

Revision Table: 8051 Serial Port Modes

Additional Information on 8051 Serial Communication

The operation of the 8051 serial port is controlled by the Special Function Register (SFR) called SCON (Serial Control). The SM0 and SM1 bits in SCON determine the communication mode:

• SM0=0, SM1=0: Mode 0
• SM0=0, SM1=1: Mode 1
• SM0=1, SM1=0: Mode 2
• SM0=1, SM1=1: Mode 3

The PCON register also influences the serial port, particularly the SMOD bit. If SMOD is set (SMOD=1), the baud rate in Mode 2 is doubled (from \( f/64 \) to \( f/32 \)). In Modes 1 and 3, SMOD doubles the Timer 1 overflow rate used for baud rate generation.

In Modes 1 and 3, Timer 1 is commonly used in 8-bit auto-reload mode (Mode 2 for Timer 1) to generate the baud rate. The reload value (TH1) is calculated based on the desired baud rate, the system clock frequency, and the state of the SMOD bit.