Department of Computer Scien

ce and Information Engineering

National Cheng Kung University, TAIWAN

HANEL

I/O BIT

MANIPULATION PROGRAMMING Reading Input Pins vs. Port

Latch

‰In reading a port

¾Some instructions read the status of port pins¾Others read the status of an internal port latch

‰Therefore, when reading ports there are two possibilities:

¾Read the status of the input pin¾Read the internal latch of the output port

‰Confusion between them is a major source of erro

rs in 8051 programming

¾Especially where external hardware is concerned