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寄存器我们是不能直接使用的。

SPI的工作方式可以从两个方面来看，一个是数据的传送，一个是数据的接收。送出数据的时候，只要将数据写入SSPBUF即可；在数据传送完成之后，SSPIF中断标志位会置1。接收的时间就比较复杂了。当有数据从SDI引脚进入单片机之后，会被存往往到SSPSR寄存器中，当完整的8个位接收完毕之后，这个字节的数据就会自动的放到SSPBUF中，一旦SSPBUF中放入新的数据，SSPCON寄存器中的BF位就会自动的设定为1，表示SSPBUF已经填入了新的值，也表示数据的接收已完成。

由于PIC16F877A通过SDO发送数据的同时，会通过SDI读入数据，当1字节的发送完成时，刚好接收1字节到SSPBUF，这时SSPBUF满，BF被置1，故可通过BF标志判断1字节是否发送完成。