Serial Interfaces - I²C

Overview

• Inter-Integrated Circuit
• Serial data bus developed by Phillips in 1982
• Mainly: Communication between a controller and peripherals on a printed circuit board
• only two signal wire are needed:
  • SCL: slave clock
  • SDA: slave data
• designed for short cable length
• technically identical bus standard: SMB (System Management Bus), TWI (Two-Wire Interface)

• typically: one master per I²C bus (multi-master systems seldom used)
• multiple slaves
• SDA and SCL lines are held on H level by a pullup resistor
• Master and slaves can pull SDA and SCL lines to GND.

• Master starts communication by sending the START sequence: SDA line is pullup down before SCL line.
• Each slave has a unique 7-bit address (MSB first)
• Master sends the address of the corresponding slave.
• The R/W bit is added to define wether data is written or read to the slave.
  • R/W = 0 → Write data to the slave
  • R/W = 1 → Read data from the slave
• The 7-bit address together with the R/W bit is defined a the 8-bit address. Make sure to find out the correct address in the datasheet.
  8-bit address = ((7-bit address) << 1) + (R/W bit)
• The slave sends an acknowledgement by pullup down the SDA line. If no slave with this address is present, the communication stops, until the master gives up.
• Depending on the R/W bit an arbitrary number of bytes is transferred. The receiver sends an acknowledgement after each byte.
• The communication is stopped by the master sending the stop condition: SCL line is released before the SDA line.

I²C Slave Example: Si7021

Si7021

• Humidity and temperature sensor
• Simple connection interface: VDD, GND, SDA, SCL
• very low standby current: 60 nA
• 7 bit address: 0x40

MSP430 Registers

I²C Library

• Download: i2c_modified.zip
• I²C with behavior with ACK is rather complex.
• Flag polling can freeze program, when slave does not respond.
• I²C library uses the WDT to generate a timeout and ensure no program freezing.
• I²C communication is aborted in case of a timeout.
• A return value is given on abort.

Check Si7021 Presence

#include <msp430.h>
#include <stdint.h>

#include "pins.h"
#include "term.h"
#include "i2c.h"

int main(void) {
    WDTCTL = WDTPW | WDTHOLD;    // stop watchdog timer

    p_setup();
    i2c_init();
    term_init();
    term_wait_and_clear();

    while (1) {
        uint8_t data[2];
        uint8_t result = i2c_read(0x40, data, 1);

        term_log_begin();
        term_print("I2C read on address 0x40: ");
        term_test_result(!result);
        term_end();

        p_delay_ms(1000);
    }
}

• A read request is attempted on address 0x40.
• If result == 1, there is a error on the I²C bus and the I²C did not respond.
• When running the program, the debug terminal output should return a green P.
• Removing the WuRx shield and disconnecting the Si7021 should result in a red X.

Reading Temperature Value

• Note!: Some WuRx shield have the HTU21D populated instead of the Si7021. Most I²C registers are identical, but the timing is slower.
• For the following programs the timing values of the HTU21D will be used to ensure compatibility.
• The temperature measurement is stated by sending the I²C command 0xF3. A pointer is to forward the message to the library.

uint8_t data = 0xF3;
i2c_write(0x40, &data, 1);

• The program must wait 70 ms to ensure complete measurement.

p_delay_ms(70);

• The result is read by an I²C interaction directly into a 16 bit integer.

uint16_t raw_data;
i2c_read(0x40, (uint8_t *) &raw_data, 2);

• Because the data is read MSB first from the sensor, but MSP430 is storing LSB first, the bytes are swapped with a MSP430 library function.

raw_data = __swap_bytes(raw_data);

• The raw data can be print over the debug terminal:

term_log_begin();
term_print("raw data T=0x");
term_hex(raw_data, 4);
term_end();

Question

Implement the program described above to read the raw temperature data. Use a calculator to convert the raw data and check plausibility.

Implement the reading of the humidity data and check the results. The HTU21D needs a delay of 36 ms to complete the measurement.

Extra: Perform the conversion on the MSP430. Use the function term_decimal() to output a fixed decimal over the debug terminal.

Solution

#include <msp430.h>
#include <stdint.h>

#include "pins.h"
#include "term.h"
#include "i2c.h"

int main(void) {
    WDTCTL = WDTPW | WDTHOLD;    // stop watchdog timer

    p_setup();
    i2c_init();
    term_init();
    term_wait_and_clear();

    while (1) {
        // I²C Check
        uint8_t data[2];
        uint8_t result = i2c_read(0x40, data, 1);

        term_log_begin();
        term_print("I2C read on address 0x40: ");
        term_test_result(!result);
        term_end();

        // Temperature
        uint8_t data2 = 0xF3;
        i2c_write(0x40, &data2, 1);
        p_delay_ms(70);
        uint16_t raw_data;
        i2c_read(0x40, (uint8_t *) &raw_data, 2);
        raw_data = __swap_bytes(raw_data);

        term_log_begin();
        term_print("raw data T=0x");
        term_hex(raw_data, 4);
        term_end();

        p_delay_ms(1000);
    }
}