4/12

USING ST62xx I/O PORTS SAFELY

If the DDR bit corresponding to the I/O pin contains a “0", the port pin is set to input. Subse-

quent read accesses to DR will return the state of the I/O pin. This means that the DR bit is ef-

fectively Read Only when the bit is in Output mode. To read the content of any DR bit set to

input mode a copy of the DR content previously written must be saved in Data RAM.

Writing to the DR bit is possible in both input and output modes. It is important to note this as

in input mode, DR is used to set the input pin characteristics (table 1). Due to the flexibility of

programming each I/O bit individually, some pins of a port may be configured to input mode

and others to output mode. Reading the DR will return some DR bit contents (from pins in

output mode) and some I/O pin states (from pins in input mode) combined in the one byte. As

will be shown in a following section this can create serious system malfunctions if care is not

taken.