C51: Reading from an Input Port

Information in this article applies to:

• C51 Version 5.50a
• C51 Version 6.01

QUESTION

How do I read from one of the I/O ports on the 8051?

ANSWER

To read an I/O port on the 8051, you must ensure that the internal latch state is at a high level. To ensure that the port bit is high, write a 1 to it. Then you may simply read the port or port bit.

For example:

sbit P1_1 = 0x91;

P1_1 = 1;    // Set port for input
var = P1_1;  // Read P1_1 into var

SEE ALSO

• C51: Writing to the Output Ports
• C51: Accessing Special Function Registers in C

Last Reviewed: Thursday, February 25, 2021