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2003 Microchip Technology Inc.

DS39582B-page 51

PIC16F87XA

4.6

Parallel Slave Port

The Parallel Slave Port (PSP) is not implemented on

the PIC16F873A or PIC16F876A.

PORTD operates as an 8-bit wide Parallel Slave Port,

or microprocessor port, when control bit PSPMODE

(TRISE<4>) is set. In Slave mode, it is asynchronously

readable and writable by the external world through RD

control input pin, RE0/RD/AN5, and WR control input

pin, RE1/WR/AN6.

The

PSP

can

directly

interface

8-bit

microprocessor data bus. The external microprocessor

can read or write the PORTD latch as an 8-bit latch.

Setting bit PSPMODE enables port pin RE0/RD/AN5 to

be the RD input, RE1/WR/AN6 to be the WR input and

RE2/CS/AN7 to be the CS (Chip Select) input. For this

functionality, the corresponding data direction bits of

the TRISE register (TRISE<2:0>) must be configured

as inputs (set). The A/D port configuration bits,

PCFG3:PCFG0 (ADCON1<3:0>), must be set to

configure pins RE2:RE0 as digital I/O.

There are actually two 8-bit latches: one for data output

and one for data input. The user writes 8-bit data to the

PORTD data latch and reads data from the port pin

latch (note that they have the same address). In this

mode, the TRISD register is ignored since the external

device is controlling the direction of data flow.

A write to the PSP occurs when both the CS and WR

lines are first detected low. When either the CS or WR

lines become high (level triggered), the Input Buffer Full

(IBF) status flag bit (TRISE<7>) is set on the Q4 clock

cycle, following the next Q2 cycle, to signal the write is

complete (Figure 4-11). The interrupt flag bit, PSPIF

(PIR1<7>), is also set on the same Q4 clock cycle. IBF

can only be cleared by reading the PORTD input latch.

The Input Buffer Overflow (IBOV) status flag bit

(TRISE<5>) is set if a second write to the PSP is

attempted when the previous byte has not been read

out of the buffer.

A read from the PSP occurs when both the CS and RD

lines are first detected low. The Output Buffer Full

(OBF)

status

flag

bit

(TRISE<6>)

cleared

immediately (Figure 4-12), indicating that the PORTD

latch is waiting to be read by the external bus. When

either the CS or RD pin becomes high (level triggered),

the interrupt flag bit PSPIF is set on the Q4 clock cycle,

following the next Q2 cycle, indicating that the read is

complete. OBF remains low until data is written to

PORTD by the user firmware.

When not in PSP mode, the IBF and OBF bits are held

clear. However, if flag bit IBOV was previously set, it

must be cleared in firmware.

An interrupt is generated and latched into flag bit

PSPIF when a read or write operation is completed.

PSPIF must be cleared by the user in firmware and the

interrupt can be disabled by clearing the interrupt

enable bit PSPIE (PIE1<7>).

FIGURE 4-10:

PORTD AND PORTE

BLOCK DIAGRAM

(PARALLEL SLAVE PORT)

Data Bus

Port

RD Port

RDx pin

One bit of PORTD

Set Interrupt Flag

PSPIF (PIR1<7>)

Read

Chip Select

Write

TTL

Note 1: I/O pins have protection diodes to VDD and VSS.

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