DM163006 Datasheet, PDF (154/320 Pages) Microchip Technology – High-Performance ROM-less Microcontrollers

PIC18C601/801
The MSSP consists of a transmit/receive shift register
(SSPSR) and a buffer register (SSPBUF). The SSPSR
shifts the data in and out of the device, MSb first. The
SSPBUF holds the data that was written to the SSPSR,
until the received data is ready. Once the 8 bits of data
have been received, that byte is moved to the SSPBUF
register. Then the buffer full detect bit, BF (SSPSTAT
register), and the interrupt flag bit, SSPIF (PIR regis-
ters), are set. This double buffering of the received data
(SSPBUF) allows the next byte to start reception before
reading the data that was just received. Any write to the
SSPBUF register during transmission/reception of data
will be ignored, and the write collision detect bit, WCOL
(SSPCON1 register), will be set. User software must
clear the WCOL bit so that it can be determined if the
following write(s) to the SSPBUF register completed
successfully.
When the application software is expecting to receive
valid data, the SSPBUF should be read before the next
byte of data to transfer is written to the SSPBUF. The
buffer full (BF) bit (SSPSTAT register) indicates when
SSPBUF has been loaded with the received data
(transmission is complete). When the SSPBUF is read,
the BF bit is cleared. This data may be irrelevant if the
SPI is only a transmitter. Generally, the MSSP interrupt
is used to determine when the transmission/reception
has completed. The SSPBUF must be read and/or
written. If the interrupt method is not going to be used,
then software polling can be done to ensure that a write
collision does not occur. Example 15-1 shows the
loading of the SSPBUF (SSPSR) for data transmission.
The SSPSR is not directly readable or writable, and
can only be accessed by addressing the SSPBUF reg-
ister. Additionally, the MSSP status register (SSPSTAT
register) indicates the various status conditions.
15.3.2 ENABLING SPI I/O
To enable the serial port, SSP enable bit, SSPEN
(SSPCON1 register), must be set. To reset or reconfig-
ure SPI mode, clear the SSPEN bit, re-initialize 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, corresponding pins 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> bit cleared
• SCK (Master mode) must have TRISC<3> bit
cleared
• SCK (Slave mode) must have TRISC<3> bit set
• RA5 must be configured as digital I/O using
ADCON1 register
• SS must have TRISA<5> bit set
Any serial port function that is not desired may be
overridden by programming the corresponding data
direction (TRIS) register to the opposite value.
EXAMPLE 15-1: LOADING THE SSPBUF (SSPSR) REGISTER
LOOP BTFSS SSPSTAT, BF
BRA LOOP
MOVF SSPBUF, W
MOVWF RXDATA
MOVF TXDATA, W
MOVWF SSPBUF
;Has data been received (transmit complete)?
;No
;WREG reg = contents of SSPBUF
;Save in user RAM, if data is meaningful
;W reg = contents of TXDATA
;New data to xmit
DS39541A-page 154
Advance Information
 2001 Microchip Technology Inc.