SLAZ497H January 2013 – March 2021 MSP430G2744
USCI Module
Functional
I2C multi-master transmit may lose first few bytes.
In an I2C multi-master system (UCMM =1), under the following conditions:
(1)the master is configured as a transmitter (UCTR =1)
AND
(2)the start bit is set (UCTXSTT =1);
if the I2C bus is unavailable, then the USCI module enters an idle state where it waits and checks for bus release. While in the idle state it is possible that the USCI master updates its TXIFG based on clock line activity due to other master/slave communication on the bus. The data byte(s) loaded in TXBUF while in idle state are lost and transmit pointers initialized by the user in the transmit ISR are updated incorrectly.
Verify that the START condition has been sent (UCTXSTT =0) before loading TXBUF with data.
Example:
#pragma vector = USCIAB0TX_VECTOR
__interrupt void USCIAB0TX_ISR(void)
{
// Workaround for USCI34
if(UCB0CTL1&UCTXSTT)
{
// TXData = pointer to the transmit buffer start
// PTxData = pointer to transmit in the ISR
PTxData = TXData; // restore the transmit buffer pointer if the Start bit is set
}
//
if(IFG2&UCB0TXIFG)
{
if (PTxData < = PTxDataEnd) // Check TX byte counter
{
UCB0TXBUF = *PTxData++; // Load TX buffer
}
else
{
UCB0CTL1 |= UCTXSTP; // I2C stop condition
IFG2 &= ~UCB0TXIFG; // Clear USCI_B0 TX int flag
__bic_SR_register_on_exit(CPUOFF); // Exit LPM0
}
}
}