Z86L972 Datasheet, PDF (31/91 Pages) Zilog, Inc. – Low-Voltage Microcontrollers

Z86L972/Z86L973/Z86L974
Low-Voltage Microcontrollers
24
Input and Output Registers
Each of the four ports (Ports 2, 4, 5, and 6) has an input register, an output regis-
ter, and associated buffer and control logic. Because there are separate input and
output registers associated with each port, writing bits defined as inputs store the
data in the output register. This data cannot be read as long as the bits are defined
as inputs. However, if the bits are reconfigured as output, the data stored in the
output register is reflected on the output pins and can then be read. This mecha-
nism allows you to initialize the outputs before driving their loads.
Because port inputs are asynchronous to the Z8 internal clock, a READ operation
could occur during an input transition. In this case, the logic level might be uncer-
tain (somewhere between a logic 1 and 0).
General Port I/O
The eight I/O lines of each port (except P43, P52, and P53) can be configured
under software control to be either input or output, independently. Bits pro-
grammed as outputs can be globally programmed as either push-pull or open-
drain. See Figure 11.
Open-Drain
I/O
Mask
Option
V_CC
Pull-Up
*
Pad
Out
Note: * Pull-up resistance is
about 200 KΩ at 2.3 V and
In
75 KΩ at 5.0 V with +50%
tolerance.
Figure 11. General Input/Output Pin
Read/Write Operations
The ports are accessed as general-purpose registers. Port registers are written by
specifying the port register as an instruction’s destination register. Writing to a port
causes data to be stored in the output register of the port, and reflected externally
on any bit configured as an output.
PS010504-1002
PRELIMINARY