PIC16F7X7 datasheet(72/276 Pages) MICROCHIP

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PIC16F7X7

DS30498C-page 70

 2004 Microchip Technology Inc.

5.6

Parallel Slave Port

The Parallel Slave Port (PSP) is not implemented on

the PIC16F737 or PIC16F767.

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 an external system using the

read control input pin RE0/RD/AN5, the write control

input pin RE1/WR/AN6 and the chip select control input

pin RE2/CS/AN7.

The PSP can directly interface to an 8-bit micro-

processor 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 (i.e., 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

(external reads) and one for data input (external

writes). The firmware writes 8-bit data to the PORTD

output data latch and reads data from the PORTD input

data 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.

An external write to the PSP occurs when the CS and

WR lines are both detected low. Firmware can read the

actual data on the PORTD pins during this time. When

either the CS or WR lines become high (level trig-

gered), the data on the PORTD pins is latched and the

Input Buffer Full (IBF) status flag bit (TRISE<7>) and

interrupt flag bit, PSPIF (PIR1<7>), are set on the Q4

clock cycle following the next Q2 cycle to signal the

write is complete (Figure 5-21). Firmware clears the

IBF flag by reading the latched PORTD data and clears

the PSPIF bit.

The Input Buffer Overflow (IBOV) status flag bit

(TRISE<5>) is set if an external write to the PSP occurs

while the IBF flag is set from a previous external write.

The previous PORTD data is overwritten with the new

data. IBOV is cleared by reading PORTD and clearing

IBOV.

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

lines are detected low. The data in the PORTD output

latch is output to the PORTD pins. The Output Buffer

Full (OBF) status flag bit (TRISE<6>) is cleared imme-

diately (Figure 5-22), indicating that the PORTD latch is

being read or has been read by the external bus. If

firmware writes new data to the output latch during this

time, it is immediately output to the PORTD pins but

OBF will remain cleared.

When either the CS or RD pins are detected high, the

PORTD outputs are disabled and 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 firmware writes new data to PORTD.

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

clear. Flag bit IBOV remains unchanged. The PSPIF bit

must be cleared by the user in firmware; the interrupt

can be disabled by clearing the interrupt enable bit,

PSPIE (PIE1<7>).

FIGURE 5-20:

PORTD AND PORTE

BLOCK DIAGRAM

(PARALLEL SLAVE PORT)

Data Bus

Port

RDx pin

Port

One bit of PORTD

Set Interrupt Flag

PSPIF (PIR1<7>)

Read

Chip Select

Write

Note: I/O pin has protection diodes to VDD and VSS.

TTL

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