Datasheet

 2012 Microchip Technology Inc. DS30684A-page 217
PIC18(L)F2X/45K50
FIGURE 16-4: SPI MASTER AND MULTIPLE SLAVE CONNECTION
16.2.1 SPI MODE REGISTERS
The MSSP module has five registers for SPI mode
operation. These are:
• MSSP STATUS register (SSPxSTAT)
• MSSP Control register 1 (SSPxCON1)
• MSSP Control register 3 (SSPxCON3)
• MSSP Data Buffer register (SSPxBUF)
• MSSP Address register (SSPxADD)
• MSSP Shift register (SSPxSR)
(Not directly accessible)
SSPxCON1 and SSPxSTAT are the control and
STATUS registers in SPI mode operation. The
SSPxCON1 register is readable and writable. The
lower six bits of the SSPxSTAT are read-only. The
upper two bits of the SSPxSTAT are read/write.
In one SPI Master mode, SSPxADD can be loaded
with a value used in the Baud Rate Generator. More
information on the Baud Rate Generator is available in
Section 16.7 “Baud Rate Generator”.
SSPxSR is the shift register used for shifting data in
and out. SSPxBUF provides indirect access to the
SSPxSR register. SSPxBUF is the buffer register to
which data bytes are written, and from which data
bytes are read.
In receive operations, SSPxSR and SSPxBUF
together create a buffered receiver. When SSPxSR
receives a complete byte, it is transferred to SSPxBUF
and the SSPIF interrupt is set.
During transmission, the SSPxBUF is not buffered. A
write to SSPxBUF will write to both SSPxBUF and
SSPxSR.
16.2.2 SPI MODE OPERATION
When initializing the SPI, several options need to be
specified. This is done by programming the appropriate
control bits (SSPxCON1<5:0> and SSPxSTAT<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)
To enable the serial port, SSPx Enable bit, SSPxEN of
the SSPxCON1 register, must be set. To reset or recon-
figure SPI mode, clear the SSPxEN bit, re-initialize the
SSPxCONx registers and then set the SSPxEN bit.
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 as
follows:
• SDI must have corresponding TRIS bit set
• SDO must have corresponding TRIS bit cleared
• SCK (Master mode) must have corresponding
TRIS bit cleared
• SCK (Slave mode) must have corresponding
TRIS bit set
•SS
must have corresponding TRIS bit set
SPI Master
SCLK
SDO
SDI
General I/O
General I/O
General I/O
SCLK
SDI
SDO
SS
SPI Slave
#1
SCLK
SDI
SDO
SS
SPI Slave
#2
SCLK
SDI
SDO
SS
SPI Slave
#3