OBDH - PSU Interface

The power supply unit (PSU) is run through an I2C bus. This bus is used to send reset commands to the PSU board, and to receive telemetry from it.

Starting up I2C

I2C is one of three modes of the USART interface on the board. In order to set it up after a reset, the following steps must be followed:

1) Select I2C mode with SWRST = 1
2) Disable the I2C module
3) Configure the I2C module with I2CEN = 0
4) Set I2CEN via software

Example Code

//I2C test code
#include <io.h>

void waitms(int time) {
  BCSCTL1=0x00; //Sets ACLK=LFXT1CLK
  TACCR0=time*4; //assumes one run of clock=0.25ms
  TACTL=0x01D2; //Resets timer, sources from ACLK/8, and sets to count up to TACCR0
  while (!(TACTL & 1));
}

char i2csend(char *data, char address, int length ){
  int i;//sets counter

  P3DIR |= 0x0A;//Configure I2C pins to output, this will need to be changed to receive data
  P3SEL |= 0x0A;//Sets pins to use peripheral module

  // Stop watchdog timer to prevent time out reset
  WDTCTL = WDTPW + WDTHOLD;

  BCSCTL2 |= SELS; //Set SMCLK to XT2 oscillator
  BCSCTL2 &= ~0x06; //Set clock divider to 1

  U0CTL |= SYNC + I2C; //Set USART 0 to I2C
  U0CTL |= MST; //Make I2C master

  U0CTL &= ~(I2CEN); //Clear I2C enable
  I2CTCTL |= 0x20; //Set I2C to use SMCLK
  I2CTCTL &= ~(I2CRM); //Set repeat mode to zero
  I2CTCTL &= ~(I2CWORD); //Set to byte mode
  I2CIE |= 0x20; //Set  transmit ready interupt
  I2CIE |= 0x08; //set register ready interupt
  I2CPSC = 0x3; //Set prescalar multiplier to 4x
  I2CSCLH = 0x8; //Set SCL high period to 8x prescalar period
  I2CSCLL = 0x8; //Set SCL low period to  8x prescalar period
  I2COA = 0x01; //Set own address to 1
  I2CSA = address; //Set slave address
  U0CTL |= I2CEN; //Renable I2C

  waitms(100); //Wait for I2C to be ready

  I2CTCTL |= I2CTRX; //Set transmit mode
  I2CNDAT = length; //Set to transmit length bytes

  for(i=0; i<length; i++){
    I2CDRB = data[i]; //Gives a data byte
    waitms(2);// I2C commands take ~1ms (Appendix D, I2C Bus Commands)
  }

  I2CTCTL |= I2CSTP; //Sets STOP condition bit
  I2CTCTL |= I2CSTT; //Initiates a start condition

    waitms(100); //Wait for I2C to be ready

  return(I2CIE);
}

int main()
{
  char data[3];
  char address; // create array with sufficient length as well as address variable
  int length;
  char interupt;

  data[0] = 0x01; // inputs 3 byte command for ttl node
  data[1] = 0x00; //the adjacent checks the voltage on panel X2
  data[2] = 0x03;
  address = 0x41; // sets slave address
  length = 3; //sets length of input

  interupt = i2csend(&data[3], address, length); // passes to send function

  P2DIR =255;
  P2OUT = interupt; //displays interupt flags

06/07/10 : The above code has only been tested using the logic analyser. It behaves in the expected manner, dropping out after transmitting the address and failing to receive the ACK bit