i2c read byte requires dummy read

Chris Leon

Hello,

Currently I find that I need to read a register twice to get the correct data. If I run a single time, the byte read is the previous byte. For example:

rtcMonth = readMonth();
rtcMonth = readMonth();
rtcYear = readYear();

gives me an rtcMonth equal to 0x01 (january) but it also gives me 0x01 for the year. If I perform a second readYear operation, rtcYear will be 0x16 (2016). My i2c is set to not generate an autostop.

My code follows:

#define i2cDelay 1000

unsigned char i2cReadByte(const unsigned char slvAddr, const unsigned char regAddr){
	char rxByte;
	while (stopBusy());
	UCB0I2CSA = slvAddr;  // set slave addr
	UCB0CTLW0 |= UCTR + UCTXSTT;  // i2c start
	while(txBusy() | startBusy());
	UCB0TXBUF = regAddr;
	while(txBusy());
	UCB0CTLW0 &= ~UCTR ;
	UCB0CTLW0 |= UCTXSTT;
	while(rxBusy() | startBusy());
	UCB0CTLW0 |= UCTXSTP;  // i2c stop
	rxByte = UCB0RXBUF;
	while (stopBusy());
	_delay_cycles(i2cDelay);
	return rxByte;

char txBusy(void){
	return !(UCB0IFG & UCTXIFG0);
}

char startBusy(void){
	return (UCB0CTLW0 & UCTXSTT);
}

char stopBusy(void){
	return (UCB0CTLW0 & UCTXSTP);
}

char rxBusy(void){
	return !(UCB0IFG & UCRXIFG0);
}

over 10 years ago

0 Clemens Ladisch over 10 years ago

Guru 321490 points

You did not mention your chip.

But in the case of the USCI module, the documentation says:

If a master wants to receive a single byte only, the UCTXSTP bit must be set while the byte is being received.

So your code would need to look like this:

...
CB0CTLW0 |= UCTXSTT;
while(startBusy());
UCB0CTLW0 |= UCTXSTP;
while(rxBusy());
rxByte = UCB0RXBUF;
...

0 Chris Leon over 10 years ago in reply to Clemens Ladisch

Prodigy 100 points

I'm using the MSP432 launchpad. I hope to convert all of this to a non-blocking, interrupt driven approach but I would like to figure out what the issue is with this code.

At some point in the past I'm pretty sure I put the stop generation before reading the rx buffer but it just hung in the rxBusy loop. I will try your suggestion, thanks for your help.

0 Clemens Ladisch over 10 years ago in reply to Chris Leon

Guru 321490 points

The MSP432 has the eUSCI module, whose documentation does not contain this sentence. But in theory, it should work the same.

0 Chris Leon over 10 years ago in reply to Clemens Ladisch

Prodigy 100 points

Your suggested fix worked. I had accidentally tried my stopBusy function instead of rxBusy before, and it would hang there.

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i2c read byte requires dummy read