PSD312 datasheet(19/85 Pages) STMICROELECTRONICS

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PSD3XX Family

10.0

I/O Port

Functions

(cont.)

10.2 Port A (PA0-PA7)

The control registers of Port A are located in CSIOPORT space; see Table 5.

10.2.1 Port A (PA0-PA7) in Multiplexed Address/Data Mode

Each pin of Port A can be individually configured. The following table summarizes what the

control registers (in CSIOPORT space) for Port A do:

NOTE: 1. Default value is the value after reset.

Default

Value

0 Value

1 Value

(Note 1)

Port A Pin Register

Sampled logic level

at pin = ‘0’

at pin = ‘1’

Port A Direction Register

Pin is configured

as input

as output

Port A Data Register

Data in DFF = ‘0’

Data in DFF = ‘1’

MCU I/O Mode

The default configuration of Port A is MCU I/O. In this mode, every pin can be set (at run-

time) as an input or output by writing to the respective pin’s direction flip-flop (DIR FF,

Figure 5A). As an output, the pin level can be controlled by writing to the respective pin’s

data flip-flop (DFF, Figure 5A). The Pin Register can be read to determine logic level of the

pin. The contents of the Pin Register indicate the true state of the PSD driving the pin

through the DFF or an external source driving the pin. Pins can be configured as CMOS

or open-drain using ST’s PSDsoft software. Open-drain pins require external pull-up

resistors.

Latched Address Output Mode

Alternatively, any bit(s) of Port A can be configured to output low-order demultiplexed

address bus bit. The address is provided by the internal PSD address latch, which latches

the address on the trailing edge of ALE/AS. Port A then outputs the desired demultiplexed

address bits. This feature can eliminate the need for an external latch (for example:

74LS373) if you have devices that require low-order latched address bits. Although any pin

of Port A may output an address signal, the pin is position-dependent. In other words, pin

PA0 of Port A may only pass A0, PA1 only A1, and so on.

Track Mode

Track Mode sets the entire port to track the signals on AD0/A0-AD7/A7, depending on

specific address ranges defined by the PAD’s CSADIN, CSADOUT1, and CSADOUT2

signals. This feature lets the user interface the microcontroller to shared external resources

without requiring external buffers and decoders. In Track Mode, Port A effectively operates

as a bi-directional buffer, allowing external MCUs or host processors to access the local

data bus. Keep the following information in mind when setting up Track Mode:

t The direction is controlled by:

• ALE/AS

• RD/E or RD/E/DS (DS on non-3X1 devices only)

• WR or R/W

• PAD outputs CSADOUT1, CSADOUT2, and CSADIN defined in PSDsoft design.

t When CSADOUT1 and ALE/AS are true, the address on AD0/A0-AD7/A7 is output on

Port A. Note: carefully check the generation of CSADOUT1 to ensure that it is stable

during the ALE/AS pulse.

t When CSADOUT2 is active and a write operation is performed, the data on the

AD0/A0-AD7/A7 input pins flows out through Port A.

t When CSADIN is active and a read operation is performed, the data on Port A flows

out through the AD0/A0-AD7/A7 pins.

t Port A is tri-stated when none of the above conditions exist.

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