PIC18C658T-I/L【PDF 49/246 页】IC MCU OTP 16KX16 CAN 68PLCC

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PIC18CXX8

DS30475A-page 142

Advanced Information

2000 Microchip Technology Inc.

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

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, 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> bit cleared

SCK (Master mode) must have TRISC<3> bit

cleared

SCK (Slave mode) must have TRISC<3> bit set

SS must have TRISC<4> bit 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.

EXAMPLE 15-1: LOADING THE SSPBUF (SSPSR) REGISTER

LOOP BTFSS SSPSTAT, BF

;Has data been received (transmit complete)?

GOTO

LOOP

;No

MOVF

SSPBUF, W

;WREG reg = contents of SSPBUF

MOVWF RXDATA

;Save in user RAM, if data is meaningful

MOVF

TXDATA, W

;W reg = contents of TXDATA

MOVWF SSPBUF

;New data to xmit

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