PIC16C765SLASHP datasheet(40/158 Pages) MICROCHIP

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PIC16C745/765

DS41124A-page 40

Advanced Information

5.6

Parallel Slave Port (PSP)

PORTD operates as an 8-bit wide Parallel Slave Port

(PSP), or microprocessor port when control bit PSP-

MODE (TRISE<4>) is set. In slave mode, it is asyn-

chronously readable and writable by the external world

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

control input pin RE1/WR/AN6.

It can directly interface to an 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) and

the

A/D

port

configuration

bits

PCFG<2:0>

(ADCON1<2:0>) must be set, which will configure pins

RE<2:0> as digital I/O.

There are actually two 8-bit latches; one for data-out

(from the PICmicro® microcontroller) and one for data

input. The user writes 8-bit data to PORTD data latch

and reads data from the port pin latch (note that they

have the same address). In this mode, the TRISD reg-

ister is ignored, since the microprocessor 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), then 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 5-9). 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>) is cleared immedi-

ately (Figure 5-10) 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 inter-

rupt flag bit PSPIF is set on the Q4 clock cycle, follow-

ing 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 5-8:

PORTD AND PORTE BLOCK

DIAGRAM (PARALLEL SLAVE

PORT)

Note:

The PIC16C745 does not provide a paral-

lel slave port. The PORTD, PORTE, TRISD

and TRISE registers are reserved. Always

maintain these bits clear.

Data Bus

PORT

RDx

PORT

One bit of PORTD

Set interrupt flag

PSPIF (PIR1<7>)

Read

Chip Select

Write

TTL

VDD

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