PIC18F2480/2580/4480/4580
18.3.3
ENABLING SPI I/O
18.3.4
TYPICAL CONNECTION
To enable the serial port, MSSP Enable bit, SSPEN
(SSPCON1<5>), must be set. To reset or reconfigure
SPI mode, clear the SSPEN bit, reinitialize the
SSPCON registers and then set the SSPEN bit. This
configures the SDI, SDO, SCK and SS pins as serial
port pins. For the pins to behave as the serial port func-
tion, some must have their data direction bits (in the
TRIS register) appropriately programmed as follows:
Figure 18-2 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
the 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 software. This leads to
three scenarios for data transmission:
• SDI is automatically controlled by the SPI module
• SDO must have TRISC<5> bit cleared
• SCK (Master mode) must have TRISC<3> bit
cleared
• Master sends data – Slave sends dummy data
• Master sends data – Slave sends data
• SCK (Slave mode) must have TRISC<3> bit set
• SS must have TRISF<7> bit set
• Master sends dummy data – Slave sends data
Note:
When the module is enabled and in
Master mode (CKE, SSPSTAT<6> = 1), a
small glitch of approximately half a TCY
may be seen on the SCK pin. To resolve
this, keep the SCK pin as an input while
setting SPEN. Then, configure the SCK
pin as an output (TRISC<3> = 0).
Any serial port function that is not desired may be
overridden by programming the corresponding Data
Direction (TRIS) register to the opposite value.
FIGURE 18-2:
SPI MASTER/SLAVE CONNECTION
SPI Master SSPM<3:0> = 00xxb
SPI Slave SSPM<3:0> = 010xb
SDO
SDI
Serial Input Buffer
(SSPBUF)
Serial Input Buffer
(SSPBUF)
SDI
SDO
SCK
Shift Register
(SSPSR)
Shift Register
(SSPSR)
LSb
MSb
MSb
LSb
Serial Clock
SCK
PROCESSOR 1
PROCESSOR 2
© 2009 Microchip Technology Inc.
DS39637D-page 195
MICROCHIP [ MICROCHIP ]
