PIC18C452T-E/L【PDF 29/279 页】IC MCU OTP 16KX16 A/D 44PLCC

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PIC18CXX2

DS39026C-page 122

2001 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. Buffer

full bit, BF (SSPSTAT<0>), 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 com-

pleted. 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 14-1 shows the loading of the

SSPBUF (SSPSR) for data transmission.

EXAMPLE 14-1:

LOADING THE SSPBUF (SSPSR) REGISTER

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)

indicates the various status conditions.

14.3.2

ENABLING SPI I/O

To enable the serial port, SSP enable bit, SSPEN

(SSPCON1<5>), must be set. To reset or reconfigure

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.

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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