Z8
®
CPU
User Manual
UM001604-0108 Input/Output Ports
61
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 writing to Port 3, data is stored in the output reg-
ister.
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.
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 (see
Figure 49 on page 62). Table 17 on page 62 lists 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 requests regardless of the configu-
ration specified in the Port 3 Mode Register.
Figure 48.
Port 3 Configuration with TTL Level Shifter and Autolatch
PIN
PIN
OUT
R
≈
500 K
Ω
Autolatch
IN
TTL Level Shifter
Port 3 Output Configuration
Port 3 Input Configuration
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