PIC16F87X
9.1
SPI Mode
FIGURE 9-1: MSSP BLOCK DIAGRAM
(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:
Internal
Data Bus
Read
Write
• Serial Data Out (SDO)
• Serial Data In (SDI)
• Serial Clock (SCK)
SSPBUF reg
SSPSR reg
Additionally, a fourth pin may be used when in a slave
mode of operation:
Shift
Clock
SDI
bit0
• 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:
SDO
Control
Enable
SS
• Master Mode (SCK is the clock output)
• Slave Mode (SCK is the clock input)
• Clock Polarity (Idle state of SCK)
SS
Edge
Select
• Data input sample phase
(middle or end of data output time)
2
Clock Select
• Clock edge
(output data on rising/falling edge of SCK)
SSPM3:SSPM0
SMP:CKE
• Clock Rate (Master mode only)
4
TMR2 output
2
2
• Slave Select Mode (Slave mode only)
Edge
Select
Figure 9-4 shows the block diagram of the MSSP mod-
ule when in SPI mode.
TOSC
Prescaler
4, 16, 64
SCK
Data to TX/RX in SSPSR
Data direction bit
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.
1999 Microchip Technology Inc.
DS30292A-page 67
MICROCHIP [ MICROCHIP ]
