Z86D990 Datasheet, PDF (32/102 Pages) Zilog, Inc. – Low-Voltage Micro controllers with ADC

Z86D990/Z86D991 OTP and Z86L99X ROM
Low-Voltage Microcontrollers with ADC
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Mode Registers
Each port has an associated Mode Register that determines the port’s functions
and allows dynamic change in port functions during program execution. Port and
Mode Registers are mapped into the Standard Register File. Because of their
close association, Port and Mode Registers are treated like any other general-pur-
pose register. There are no special instructions for port manipulation. Any instruc-
tion that addresses a register can address the ports. Data can be directly
accessed in the Port Register, with no extra moves.
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 the user 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 12.
PS003807-1002
PRELIMINARY