
216
Agere Systems Inc.
T8302 Internet Protocol Telephone
Advanced RISC Machine (
ARM
)
Data Sheet
July 2001
16 Parallel Peripheral Interface (PPI)
(continued)
16.1.1 PPI Pin Configuration on Reset
After reset, all PPI pins are configured as inverting inputs with pull-ups enabled.
16.1.2 Procedure for Writing to an Output Pin
1. Program the
PPI data direction register
for the pin as an output.
2. Program the
PPI
port
sense register
for the output as open-drain or direct-drive.
3. Program the
PPI port polarity register
for the output as inverted or noninverted (relative to the
PPI port data
register
).
4. Write a value in the
PPI port data register
,
PPI port data clear register
, or
PPI port data set register
to
specify the output level. If the corresponding
PPI port polarity register
bit is 1, a 1 in the
PPI port data reg-
ister
causes the output pin to drive high if it is programmed as a direct-drive output or causes the output pin to
go to high impedance if it is programmed as an open-drain output. Conversely, if the corresponding
port
polarity register
bit is 0, a 1 in the
PPI port data register
causes both direct-drive and open-drain output
pins to drive low.
16.1.3 Procedure for Reading from an Input Pin
1. Program the
PPI port data direction register
for the pin as an input.
2. Set the
PPI port sense register
to 0.
3. Program the
PPI pull-up enable register
if a pull-up resistor is desired on the I/O.
4. Program the
PPI port polarity register
to indicate whether the level on the pin is inverted before going to the
PPI port data register
.
5. Read the
PPI port data register
.
Note:
Reading the
PPI port data clear register
or the
PPI port data set register
has the same effect as reading
the
PPI port data register
.
16.1.3.1 Additional Read/Write Notes
I
If the PPI bit is configured as an input, a high value on the pin is read as 1 in the
PPI port data register
if the cor-
responding bit of the
PPI port polarity register
is 1. Conversely, a low value on the input is read as 1 if the cor-
responding bit of the
PPI port polarity register
is 0.
I
When the
PPI port data register
is written, only the chip pins
configured as outputs are modified; those config-
ured as inputs are unaffected.
I
Input pins are asynchronous and are sampled at the system clock rate. In order for an input signal to be regis-
tered, it must have a minimum pulse-width of two system clock periods; see Figure 29 below. The
CLK
in this fig-
ure is the
SYSTEM_CLK
as defined by the clock selected in the reset/cock management section
(see Reset/
Clock Management on page 29)
.