Interfacing 8251 USART with 8085 Microprocessor

The 8251 chip is Universal Synchronous Asynchronous Receiver Transmitter (USART). It acts as a mediator between the microprocessor and peripheral devices. It converts serial data to parallel form and vice versa. This chip is 28 pin DIP.

The pin description of 8251A chip

Now let us see the functional block diagram of the 8251 chip.

There are five different sections in this diagram. These sections are as follows −

• Read/ Write control logic

• Transmitter 

• Receiver

• Data Bus Buffer

• Modem Control.

Interfacing 8251 with8085

Now let us see how 8251 can be interfaced with 8085. In the diagram, we can see that eight data lines D_7-0 are connected to the data bus of the microprocessor. And also the RD and WR of the 8251 are also connected with the RD and RD of 8051. The 8251 is getting the clock from the CLK OUT pin of 8085. And the RESET is also connected to the RESET OUT pin of the microprocessor.

The C/D  pin is used to select either control register or data register. This pin is connected to the A₀ pin of 8085. The CS pin of 8251 is attached to the output of an address decoder circuit. The address decoder uses A₇ to A₁ lines of the microprocessor. In this diagram the CS will be enabled when A₇ and A₄ is at logic 1, and all other lines are at logic 0.

From the following table, we can see how to read or write data word, read the status word and write control word.