MSP430 --> ADS1298 SPI UCB0RXBUF Problem

RDW

Other Parts Discussed in Thread: MSP430F2274, ADS1298, ADS1293

I am currently working with data converter ADS1298 (self made PCB board). The MSP430F2274 from http://www.ti.com/tool/ez430-rf2500 going to program the ADS1298. I have a problem on reading Register ID of the ADS1298.

I can manage to send data via SPI sending :0x11 (Stop Data Continuous) , 0x20 (ID register address) , 0x00 (number of register to read), then 0xAA (Dummy code).

The problem is, I suppose to get the ID = 0x92(10010010), but the the MSP430 always receive 0x0F (the picture below). It seems like the MSP430 outputting 0x0F to the SIMO pin.

I tried to disconnect MSP430 SOMI pin to the ADS1298 and directly connect the output of ADS1298 DOUT pin to the oscilloscope. And the result is fine 0x92(10010010) (picture below)

I suspect the problem is because the MSP430 running with 3.6V, and the ADS1298 is powered with 1.8V DVDD which output the SPI in 1.8V as well.

But, I have checked the MSP430 datasheet, the positive threshold voltage is 1.25-2.25V. As a reference I copy part of my SPI code I used here.

//-------SPI----------------

unsigned char ADS1298_SPI_Data(unsigned char data){ //Write to Register(WREG)
UCB0CTL0 &= ~UCCKPH;
UCB0CTL0 &= ~UCCKPL;

IFG2 &= ~UCB0RXIFG; //RX flag clear

UCB0TXBUF = data;
while(!(IFG2 & UCB0RXIFG)); //while loop check
IFG2 &= ~UCB0RXIFG; //TX & RX flag clear
IFG2 &= ~UCB0TXIFG;

Clear_ADS1298_Enable(); //CLEAR ADS1298 enable

return UCB0RXBUF;
}

Am I missing something in the program? or is it because of the voltage problem?

Thanks..

over 11 years ago

0 Clemens Ladisch over 11 years ago

Guru 322000 points

1.8 V is not enough for a threshold of 2.25 V.

0 RDW over 11 years ago in reply to Clemens Ladisch

Prodigy 65 points

but from the datasheet. the threshold voltage in the range of 1.25-2.25V

0 Clemens Ladisch over 11 years ago in reply to RDW

Guru 322000 points

The MIN/MAX specifications of the threshold in the datasheet just say that the threshold is never smaller than 1.35 V, and never larger than 2.25 V. However, there is no guarantee for the actual threshold value, it is just somewhere in this interval. In your case, it is obviously above 1.8 V.

0 Todd Anderson78572 over 11 years ago in reply to Clemens Ladisch

Guru 14640 points

You may want to look at http://www.ti.com/tool/ADS1293EVM?keyMatch=ads1293evm&tisearch=Search-EN-Everything

Although it is ADS1293 and not ADS1298, the EVM looks to be configured for digital 3.3 volt operation. Is there a reason you cannot do the same?

Otherwise, you may need to add some voltage translation into your circuit.

0 Dennis Eichmann over 11 years ago

Guru 74080 points

I hope there is some current limiting between the MSP and the ADS.
3.6V -> 1.8V
The clamping diodes will become active.

0 Jens-Michael Gross over 11 years ago in reply to Dennis Eichmann

Guru 227245 points

The current-limiting resistor and the clamping diodes (if any on the ADC side) can be (ab) used to tweak the signal levels. instead of 0V/1.8V you can use a pull-up to raise the MSP-visible signal levels to e.g. 1V/2.8V by pulling the MSP side up to VCC with a current that results in a voltage drop of (in this case) 1V through the current-limiting resistor.
Some math required, but usually, a combination of pull-up and series resistor can be found to move the signal levels in a proper range.
Of course this adds additional cross-current (depending on the resistor values). But spares a dedicated level-changer.

0 RDW over 11 years ago in reply to Jens-Michael Gross

Prodigy 65 points

thanks for the reply.

I will change the DVDD for the ADS1298 to 2.5V, which is more than enough for the MSP430 positive threshold.

The other question is, why is my MSP430 always reading 0x0F on the SOMI pin? Is it because when it reads nothing on the pin, the SOMI pin will output 0x0F?

0 Jens-Michael Gross over 11 years ago in reply to RDW

Guru 227245 points

0xff, not 0x0f.
In master mode, the SOMI pin is high-impedance input. Unless you enabled the pull-up resistor. So in theory, it may have any state or even oscillate with incoming radio waves. Likely, it catches cross-talk form the clock signal, catching 0x00 or 0xff depending on clock phase and polarity. That is, as long as nobody sends a real signal.
However, if you didn't select the PIN for USCI module use,a s you don't need the SOMI input, then the sewered internal logic will keep the module input signal on '1' level, resulting in 0xFF being read.

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MSP430 --> ADS1298 SPI UCB0RXBUF Problem