PICDEM2 datasheet(21/60 Pages) MICROCHIP

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PICDEM 2 User’s Guide

DS30374D-page 16

 2001 Microchip Technology Inc.

5.1 SSP Module

This section presents the flowcharts to implement a software I2C master

mode in hardware. For MSSP operation, refer to Section 5.2.

5.1.1

Main Routine

The main routine is actually preceded by several register file, bit and vector

assignments (Figure 5.2). The program begins by configuring PORTB as

outputs and PORTC as inputs. The output data value will be read from

PORTB, so it is initialized to one.

PORTC is cleared to set SCL and SCA low when not in tri-state. This is

important as I2C is an open-drain bus. That is, only the zero values are

actually driven to the bus. Ones are transmitted by making the pin an input

(tristate) and allowing the pullup resistor to pull the line high. Data is

transmitted in this manner by initializing the SCL and SDA port pins to zero

and then writing the output value to the corresponding TRIS bit. A one written

to the bit causes the pin to tristate pulled high and a zero causes it to become

an output, driven low.

Interrupts are not used in this program, so they are disabled. Also, the SSP is

configured to enable I2C master mode.

The write/read loop begins with the initialization of the EEPROM address to

00h, and the device address to A0h. Then the write loop executes by writing

the output data value to each address in the EEPROM (Figure 5.3). After the

write loop, the EEPROM address and device address are re-initialized for

read. The read loop executes by reading data values for each EEPROM

address and verifying their accuracy (Figure 5.4).

After the read loop, the output data value is incremented by one and this

value displayed, via PORTB, on the LEDs. Then the write/read loop begins

again.

If an error is encountered when writing or reading the EEPROM, or if an

EEPROM value read is incorrect, the program jumps to the error loop, which

flashes the LEDs (Figure 5.5).

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