PIC16C7X Datasheet, PDF (86/288 Pages) Microchip Technology – 8-Bit CMOS Microcontrollers with A/D Converter

PIC16C7X
Applicable Devices
72 73 73A 74 74A 76 77
To enable the serial port, SSP Enable bit, SSPEN
(SSPCON<5>) must be set. To reset or reconfigure SPI
mode, clear bit SSPEN, re-initialize the SSPCON reg-
ister, and then set bit SSPEN. This configures the SDI,
SDO, SCK, and SS pins as serial port pins. For the pins
to behave as the serial port function, they must have
their data direction bits (in the TRISC register) appro-
priately programmed. That is:
• SDI must have TRISC<4> set
• SDO must have TRISC<5> cleared
• SCK (Master mode) must have TRISC<3>
cleared
• SCK (Slave mode) must have TRISC<3> set
• SS must have TRISA<5> set
Any serial port function that is not desired may be over-
ridden by programming the corresponding data direc-
tion (TRIS) register to the opposite value. An example
would be in master mode where you are only sending
data (to a display driver), then both SDI and SS could
be used as general purpose outputs by clearing their
corresponding TRIS register bits.
Figure 11-10 shows a typical connection between two
microcontrollers. The master controller (Processor 1)
initiates the data transfer by sending the SCK signal.
Data is shifted out of both shift registers on their pro-
grammed clock edge, and latched on the opposite edge
of the clock. Both processors should be programmed to
same Clock Polarity (CKP), then both controllers would
send and receive data at the same time. Whether the
data is meaningful (or dummy data) depends on the
application firmware. This leads to three scenarios for
data transmission:
• Master sends data — Slave sends dummy data
• Master sends data — Slave sends data
• Master sends dummy data — Slave sends data
The master can initiate the data transfer at any time
because it controls the SCK. The master determines
when the slave (Processor 2) is to broadcast data by
the firmware protocol.
In master mode the data is transmitted/received as
soon as the SSPBUF register is written to. If the SPI is
only going to receive, the SCK output could be disabled
(programmed as an input). The SSPSR register will
continue to shift in the signal present on the SDI pin at
the programmed clock rate. As each byte is received, it
will be loaded into the SSPBUF register as if a normal
received byte (interrupts and status bits appropriately
set). This could be useful in receiver applications as a
“line activity monitor” mode.
In slave mode, the data is transmitted and received as
the external clock pulses appear on SCK. When the
last bit is latched the interrupt flag bit SSPIF (PIR1<3>)
is set.
The clock polarity is selected by appropriately program-
ming bit CKP (SSPCON<4>). This then would give
waveforms for SPI communication as shown in
Figure 11-11, Figure 11-12, and Figure 11-13 where
the MSB is transmitted first. In master mode, the SPI
clock rate (bit rate) is user programmable to be one of
the following:
• FOSC/4 (or TCY)
• FOSC/16 (or 4 • TCY)
• FOSC/64 (or 16 • TCY)
• Timer2 output/2
This allows a maximum bit clock frequency (at 20 MHz)
of 5 MHz. When in slave mode the external clock must
meet the minimum high and low times.
In sleep mode, the slave can transmit and receive data
and wake the device from sleep.
FIGURE 11-10: SPI MASTER/SLAVE CONNECTION (PIC16C76/77)
SPI Master SSPM3:SSPM0 = 00xxb
SDO
Serial Input Buffer
(SSPBUF)
SPI Slave SSPM3:SSPM0 = 010xb
SDI
Serial Input Buffer
(SSPBUF)
Shift Register
(SSPSR)
MSb
LSb
PROCESSOR 1
SDI
Serial Clock
SCK
SDO
Shift Register
(SSPSR)
MSb
LSb
SCK
PROCESSOR 2
DS30390E-page 86
© 1997 Microchip Technology Inc.