PIC16F877ATISLASHSP datasheet(77/234 Pages) MICROCHIP

77 / 234 page

 2003 Microchip Technology Inc.

DS39582B-page 75

PIC16F87XA

9.3.3

ENABLING SPI I/O

To enable the serial port, SSP Enable bit, SSPEN

(SSPCON<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

overridden by programming the corresponding data

direction (TRIS) register to the opposite value.

9.3.4

TYPICAL CONNECTION

Figure 9-2 shows a typical connection between two

microcontrollers. The master controller (Processor 1)

initiates the data transfer by sending the SCK signal.

Data is shifted out of both shift registers on their

programmed clock edge and latched on the opposite

edge of the clock. Both processors should be

programmed to the same Clock Polarity (CKP), then

both controllers would send and receive data at the

same time. Whether the data is meaningful (or dummy

data) depends on the application software. This leads

to three scenarios for data transmission:

• Master sends data – Slave sends dummy data

• Master sends data – Slave sends data

• Master sends dummy data – Slave sends data

FIGURE 9-2:

SPI MASTER/SLAVE CONNECTION

Serial Input Buffer

(SSPBUF)

Shift Register

(SSPSR)

MSb

LSb

SDO

SDI

PROCESSOR 1

SCK

SPI Master SSPM3:SSPM0 = 00xxb

Serial Input Buffer

(SSPBUF)

Shift Register

(SSPSR)

LSb

MSb

SDI

SDO

PROCESSOR 2

SCK

SPI Slave SSPM3:SSPM0 = 010xb

Serial Clock

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