PIC18F2420/2520/4420/4520
SSPSR is the shift register used for shifting data in or
out. SSPBUF is the buffer register to which data bytes
are written to or read from.
17.3.1
REGISTERS
The MSSP module has four registers for SPI mode
operation. These are:
In receive operations, SSPSR and SSPBUF together
create a double-buffered receiver. When SSPSR
receives a complete byte, it is transferred to SSPBUF
and the SSPIF interrupt is set.
• MSSP Control Register 1 (SSPCON1)
• MSSP Status Register (SSPSTAT)
• Serial Receive/Transmit Buffer Register
(SSPBUF)
During transmission, the SSPBUF is not double-
buffered. A write to SSPBUF will write to both SSPBUF
and SSPSR.
• MSSP Shift Register (SSPSR) – Not directly
accessible
SSPCON1 and SSPSTAT are the control and status
registers in SPI mode operation. The SSPCON1 regis-
ter is readable and writable. The lower 6 bits of the
SSPSTAT are read-only. The upper two bits of the
SSPSTAT are read/write.
REGISTER 17-1: SSPSTAT: MSSP STATUS REGISTER (SPI MODE)
R/W-0
SMP
R/W-0
CKE(1)
R-0
D/A
R-0
P
R-0
S
R-0
R-0
UA
R-0
BF
R/W
bit 7
bit 0
Legend:
R = Readable bit
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
-n = Value at POR
bit 7
SMP: Sample bit
SPI Master mode:
1= Input data sampled at end of data output time
0= Input data sampled at middle of data output time
SPI Slave mode:
SMP must be cleared when SPI is used in Slave mode.
bit 6
CKE: SPI Clock Select bit(1)
1= Transmit occurs on transition from active to Idle clock state
0= Transmit occurs on transition from Idle to active clock state
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
D/A: Data/Address bit
Used in I2C™ mode only.
P: Stop bit
Used in I2C mode only. This bit is cleared when the MSSP module is disabled, SSPEN is cleared.
S: Start bit
Used in I2C mode only.
R/W: Read/Write Information bit
Used in I2C mode only.
UA: Update Address bit
Used in I2C mode only.
BF: Buffer Full Status bit (Receive mode only)
1= Receive complete, SSPBUF is full
0= Receive not complete, SSPBUF is empty
Note 1: Polarity of clock state is set by the CKP bit (SSPCON1<4>).
DS39631E-page 162
© 2008 Microchip Technology Inc.
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