PIC18F23K20-ESLASHML datasheet(197/456 Pages) MICROCHIP

197 / 456 page

 2010 Microchip Technology Inc.

DS41303G-page 197

PIC18F2XK20/4XK20

17.3.3

ENABLING SPI I/O

To enable the serial port, SSP Enable bit, SSPEN of

the SSPCON1 register, must be set. To reset or recon-

figure SPI mode, clear the SSPEN bit, reinitialize 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 as follows:

• SDI is automatically controlled by the SPI module

• SDO must have corresponding TRIS bit cleared

• SCK (Master mode) must have corresponding

TRIS bit cleared

• SCK (Slave mode) must have corresponding

TRIS bit set

•SS must have corresponding TRIS 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.

17.3.4

TYPICAL CONNECTION

Figure 17-2 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 pro-

grammed 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 17-2:

SPI MASTER/SLAVE CONNECTION

Serial Input Buffer

(SSPBUF)

Shift Register

(SSPSR)

MSb

LSb

SDO

SDI

Processor 1

SCK

SPI Master SSPM<3:0> = 00xxb

Serial Input Buffer

(SSPBUF)

Shift Register

(SSPSR)

LSb

MSb

SDI

SDO

Processor 2

SCK

SPI Slave SSPM<3:0> = 010xb

Serial Clock

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