PIR1 Register
The PIR1 register contains the individual flag bits for the peripheral
interrupts.

PIR1 REGISTER
PSPIF ADIF RCIF TXIF SSPIF CCP1IF TMR2IF TMR1IF

SSPIF:Synchronous Serial Port Interrupt Flag bit
1 = The SSP interrupt condition has occurred and must be cleared in software
before returning from the interrupt Service Routine. The condition that will
set this are:
SPI-A transmission/reception has taken,place.
I2C Slave - A transmission/reception has taken place.
I2C Master
0 = No SSP interrupt condition has occurred.

SSPCON1: MSSP CONTROL REGISTER1
WCOL SSPOV SSPEN CKP SSPM3 SSPM2 SSPM1 SSPM0

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.

CKE:SPI Clock Select bit

1 = Transmit occurs on transition from active to Idle clock state

0 = Transmit occurs on transition from Idle to active clock state

BF:Buffer Full Status bit(Recevie mode only)

1 = Receive complete,SSPBUF is full

0 = Receive not complete,SSPBUF is empty

SSPSR寄存器我們是不能直接使用的。

SPI的工作方式可以從兩個方面來看，一個是數(shù)據(jù)的傳送，一個是數(shù)據(jù)的接收。送出數(shù)據(jù)的時候，只要將數(shù)據(jù)寫入SSPBUF即可；在數(shù)據(jù)傳送完成之后，SSPIF中斷標(biāo)志位會置1。接收的時間就比較復(fù)雜了。當(dāng)有數(shù)據(jù)從SDI引腳進(jìn)入單片機(jī)之后，會被存往往到SSPSR寄存器中，當(dāng)完整的8個位接收完畢之后，這個字節(jié)的數(shù)據(jù)就會自動的放到SSPBUF中，一旦SSPBUF中放入新的數(shù)據(jù)，SSPCON寄存器中的BF位就會自動的設(shè)定為1，表示SSPBUF已經(jīng)填入了新的值，也表示數(shù)據(jù)的接收已完成。

由于PIC16F877A通過SDO發(fā)送數(shù)據(jù)的同時，會通過SDI讀入數(shù)據(jù)，當(dāng)1字節(jié)的發(fā)送完成時，剛好接收1字節(jié)到SSPBUF，這時SSPBUF滿，BF被置1，故可通過BF標(biāo)志判斷1字節(jié)是否發(fā)送完成。