Read/write operations, Port a – Zilog Z8PE002 User Manual

Page 34

Z8PE002

Z8Plus OTP Microcontroller

ZiLOG

P R E L I M I N A R Y

DS008700-Z8X0799

for that bit position contains the current synchronized input
value.

For port bits configured as an output by means of the di-
rectional control register, the value held in the correspond-
ing bit of the Output Value Register is driven directly onto

the output pin. The opposite register bit for a given pin (the
output register bit for an input pin and the input register bit
for an output pin) holds their previous value. These bits are
not changed and do not exhibit any effect on the hardware.

READ/WRITE OPERATIONS

The control for each port is done on a bit-by-bit basis. All
bits are capable of operating as inputs or outputs, depending
on the setting of the port’s directional control register. If
configured as an input, each bit is provided a Schmitt-trig-
ger. The output of the Schmitt-trigger is latched twice to
perform a synchronization function, and the output of the
synchronizer is fed to the port input register, which can be
read by software.

WRITE

to a port input register carries the effect of up-

dating the contents of the input register, but subsequent

READs

do not necessarily return the same value that was

written. If the bit in question is defined as an input, the input
register for that bit position contains the current synchro-
nized input value.

WRITEs

to that bit position are overwrit-

ten on the next clock cycle with the newly sampled input
data. However, if the particular bit is programmed as an out-
put, the input register for that bit retains the software-up-
dated value. The port bits that are programmed as outputs
do not sample the value being driven out.

Any bit in either port can be defined as an output by setting
the appropriate bit in the directional control register. In this
instance, the value held in the appropriate bit of the port out-
put register is driven directly onto the output pin.

Note:

The preceding result does not necessarily reflect the actual
output value. If an external error is holding an output pin ei-
ther High or Low against the output driver, the software

READ

returns the

requested

value, not the actual state

caused by the contention. When a bit is defined as an output,

the Schmitt-trigger on the input is disabled to save power.

Updates to the output register take effect based on the timing
of the internal instruction pipeline; however, this timing is
referenced to the rising edge of the clock. The output reg-
ister can be read at any time, and returns the current output
value that is held. No restrictions are placed on the timing
of

READs

and/or

WRITEs

to any of the port registers with

respect to the others.

Note: Care should be taken when updating the directional con-

trol and special function registers.

When updating a directional control register, the special
function register (

SFR

) should first be disabled. If this pre-

caution is not taken, unpredicted events could occur as a re-
sult of the change in the port I/O status. This precaution is
especially important when defining changes in Port B, as
the unpredicted event referred to above could be one or
more interrupts. Clearing of the

SFR

first step in configuring the port, while setting the

SFR

reg-

ister should be the final step in the port configuration pro-
cess. To ensure unpredictable results, the

SFR

should not be written until the pins are being driven appro-
priately, and all initialization is completed.

PORT A

Port A is a general-purpose port. Figure 27 features a block
diagram of Port A. Each of its lines can be independently
programmed as input or output via the Port A directional
control register (

PTADIR

0D2H

) as seen in Figure 26. A

bit set to a

PTADIR

configures the corresponding bit in

Port A as an output, while a bit cleared to

configures the

corresponding bit in Port A as an input.

The input buffers are Schmitt-triggered. Bits programmed
as outputs can be individually programmed as either push-

pull or open-drain by setting the corresponding bit in the
special function register (

PTASFR

, Figure 26).

Figure 27. Port A Directional Control Register

D7 D6 D5 D4 D3 D2 D1 D0

0 = Input

1 = Output

PTADIR Register