
Z8 Microcontrollers
ZiLOG
I/O Ports
UM001600-Z8X0599
5-17
5.5.2 Read/Write Operations
Port 3 is accessed as a General-Purpose Register P3
(03H). Port 3 is written by specifying P3 as an instruction’s
destination register. However, Port 3 outputs cannot be
written to if they are used for special functions. When writ-
ing to Port 3, data is stored in the output register.
Port 3 is read by specifying P3 as the source register of an
instruction. When reading from Port 3, the data returned is
both the data on the input pins and in the output register.
5.5.3 Special Functions
Special functions for Port 3 are defined by programming
the Port 3 Mode Register. By writing 0s in bit 6 through bit
1, lines P37–P30 are configured as input/output pairs (Fig-
ure 5-22). Table 5-1 shows available functions for Port 3.
The special functions indicated in the figure are discussed
in detail in their corresponding sections in this manual.
Port 3 input lines P33–P30 always function as interrupt re-
quests regardless of the configuration specified in the Port
3 Mode Register.
Figure 5-21. Port 3 Configuration with TTL Level Shifter and Auto Latch
PIN
PIN
OUT
R
≈
500 K
Auto Latch
IN
TTL Level Shifter
Port 3 Output Configuration
Port 3 Input Configuration