
1999 Microchip Technology Inc.
Preliminary
DS30221A-page 57
PIC16F872
9.1
SPI Mode
The SPI mode allows 8 bits of data to be synchronously
transmitted and received simultaneously. All four
modes of SPI are supported. To accomplish communi-
cation, typically three pins are used:
Serial Data Out (SDO)
Serial Data In (SDI)
Serial Clock (SCK)
Additionally, a fourth pin may be used when in a slave
mode of operation:
Slave Select (SS)
When initializing the SPI, several options need to be
specified. This is done by programming the appropriate
control bits (SSPCON<5:0> and SSPSTAT<7:6>).
These control bits allow the following to be specified:
Master Mode (SCK is the clock output)
Slave Mode (SCK is the clock input)
Clock Polarity (Idle state of SCK)
Data input sample phase
(middle or end of data output time)
Clock edge
(output data on rising/falling edge of SCK)
Clock Rate (Master mode only)
Slave Select Mode (Slave mode only)
Figure 9-4
shows the block diagram of the MSSP mod-
ule when in SPI mode.
FIGURE 9-1:
MSSP BLOCK DIAGRAM
(SPI MODE)
To enable the serial port, MSSP Enable bit, SSPEN
(SSPCON<5>) must be set. To reset or reconfigure SPI
mode, clear bit SSPEN, re-initialize the SSPCON reg-
isters, 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, some must
have their data direction bits (in the TRIS register)
appropriately programmed. That is:
SDI is automatically controlled by the SPI module
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.
Read
Write
Internal
Data Bus
SSPSR reg
SSPBUF reg
SSPM3:SSPM0
bit0
Shift
Clock
SS Control
Enable
Edge
Select
Clock Select
TMR2 output
2
T
OSC
Prescaler
4, 16, 64
Edge
Select
2
4
Data to TX/RX in SSPSR
Data direction bit
2
SMP:CKE
SDI
SDO
SS
SCK