PIC16F87X
3.6
Parallel Slave Port
FIGURE 3-9: PORTD AND PORTE BLOCK
DIAGRAM (PARALLEL SLAVE
PORT)
The Parallel Slave Port is not implemented on the
PIC16F873 or PIC16F876.
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 and WR control input pin
RE1/WR.
Data Bus
D
Q
WR
PORT
RDx
pin
CK
TTL
Q
D
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 to be the RD input, RE1/WR
to be the WR input and RE2/CS 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 con-
figuration bits PCFG3:PCFG0 (ADCON1<3:0>) must
be set to configure pins RE2:RE0 as digital I/O.
RD
PORT
EN
EN
One bit of PORTD
Set interrupt flag
PSPIF (PIR1<7>)
There are actually two 8-bit latches. One for data-out
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 micro-
processor is controlling the direction of data flow.
Read
RD
CS
WR
TTL
Chip Select
TTL
Write
TTL
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 3-10). 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.
Note: I/O pin has protection diodes to VDD and VSS.
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 3-11) 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>).
DS30292B-page 38
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
