INPUT/OUTPUT PORTS2035.5 Port 3Port 3 can function as a quasi-bidirectional port capable of handling input and output of 8-bitdata in the circuit configuration outlined in Figure 5-10, and can also be used as a CPU controlpin.When port 3 is used as a quasi-bidirectional port, all functions are identical to those describedfor port 1. And when used as a CPU control pin, the port is used after first setting “1” data inthe port latch. Note that if the port is used with “0” port latch data, the CPU control signal isANDed (logical product) with the port “0” data, resulting in the CPU control signal remainingat “0” level.To change port 3 from a quasi-bidirectional input port to a high impedance input port, “1” isset in bit 3 (P3HZ) of the I/O control register (IOCON 0F8H). The output driver circuit is thusdisconnected from the port pin (floating pin status) and the port becomes a high impedanceinput port. The signal levels applied to high impedance input ports are normal “0” and “1” levelsignals. The pins cannot be used in open status.When port outputs are floated in CPU power down mode (PD, HPD), normal “0” and “1” levelsignals are applied to pins 2 thru 5 of port 3, and pins 0, 1, 6, and 7 may be either open, orundefined within the –0.5 to V CC+0.5V range. The CPU control function pins are listed inTable 5-5, and the port 3 pin table is given in Table 5-6.QDREADINTERNALBUSWP3MODIFYQD CCONTROLP1VCCP2 P3PORT 3NDATA OUTDATA INFigure 5-10 Port 3 internal equivalent circuit
