Working Of 8251A USART with Block Diagram

Table of Content

• Definition of 8251A USART
• Block Diagram [Architecture] of 8251A USART
• Explanation of Working of 8251A USART
• Control Words of 8251A USART
• How to transmit and receive Serial data in 8251A USART?

Definition of 8251A USART

8251A USART stands for Universal Synchronous Asynchronous Receiver Transmitter which acts as a mediator between a microprocessor and peripheral devices. 

• It helps in Serial data communication. 
• It receives parallel data from CPU and transmits serial data after conversion. 
• Similarly, it receives serial data from the Input/Output device and transmits parallel data after conversion.

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Block Diagram [Architecture] of 8251A USART

Explanation of Working of 8251A USART

The seven different blocks present in the USART are:

1. Data Bus Buffer
2. Read/Write Control Logic
3. Modem Control [Modulator/Demodulator]
4. Transmit Buffer
5. Transmit Control
6. Receive Buffer
7. Receive Control

A. Data Bus Buffer:

Data Bus Buffer help in interfacing the internal data bus of 8251A USART to system data bus. 

• D0-D7 = Bidirectional data bus to carry Control Word.

B. Read/Write Control Logic:

This block is the Control Block for the overall device. It controls the overall work of the system by selecting the operation to be done. 

• RESET: It is the input terminal that resets the status word. 
• CLK: It is an input terminal that generates the device timing signal. 
• WR: It is an input terminal, an active low signal that transmits data and control word from CPU to 8251A USART.
• WR: It is an active low input terminal that receives a signal for reading received data. 
• C/D: Selects data or Control word as a command. If it's value is 0, data is accessed else command is accessed. 
• CS: It is an active low chip select signal. 

Operation Between CPU and 8251A USART:

CS	C/D	RD	WR	operation
0	1	0	1	Status Word->CPU
0	1	1	0	" Control Word <- CPU"
0	0	0	1	Data->CPU
0	0	1	0	Data<-cpu>

C. Modem Control [Modulator/ Demodulator]:

It converts Analog Signal to Digital Signal and vice versa. Its Pins are:

• DSR: Data Set Ready Signal is an Input Signal.
• DTR: Data Terminal Ready Signal is an Output Signal.
• CTS: It is an Input Signal to control transmit signal.
• RTS: It is an Output signal to set the status of Modem.

D. Transmit Buffer:

It helps to transfer data after conversion.

• TXD: It is an output signal. If TXD = 1, the Transmitter will transmit data. 

E. Transmit Control:

It helps to control data transmission with the help of pins. 

• TXRDY: Output signal which indicates that the 8251A USART is ready to transmit data. 
• TXEMPTY: Output signal that indicates the transmitter is empty now.
• TXC: Clock Input Signal, active low which controls data transmission rate. 

F. Receive Buffer:

It helps to receive data. 

• RXD: It is an input signal which receives data. 

G. Receive Control:

It controls the receiving data. 

• RXRDY: Output signal that indicates 8251A is ready to receive data. 
• RXC: It is an active low clock input signal which controls the rate of transmission of received data.
• SYNDET/BD: It is an input/output terminal. Its modes are:
• Synchronous Mode: Forces 8251A to start receiving data character.
• Asynchronous Mode: It detects the Break Character.

Control Words of 8251A USART

There are two control words in 8251A USART. They are as follows:

1. Command Word
2. Status Word

A. Command Word

B. Status Word

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How to transmit and receive Serial data in 8251A USART?

A. Transmitting Serial Data

The steps for transmitting serial data in 8251A USART are:

1. Point to the Status Register.
2. Read Status Word.
3. Check the status of DSR(Data Set Ready) and TXRDY(Transmit Ready).
4. If yes, Transmit data else Jump to step 1.
5. Stop.

B. Receiving Serial Data

The steps for receiving serial data in 8251A USART are:

1. Point to the Status Register.
2. Read Status Word.
3. Check the status of RXRDY(ReceiveReady).
4. If yes, Receive data else Jump to step 1.
5. Stop.

Conclusion:

So these are all you need to know about the 8251A USART as this post contains about the Definition, Block diagram, Explanation of Block Diagram and control words in USART.

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