MSP430FR6989: Use P1.1 as input for External Voltage Reference (VeRef+) on launchpad

Harry

I am not trying to use the P1.1 as ADC channel. I have connected the Vref from REF3320AIDBZT on my sensor board which gives a constant 2048mV to the P1.1 and I am trying to see if I am getting the correct value on my VeRef (P1.1). Below is the output of pins P1.1, P8.4 and P8.5. I have connected VeRef 2048mV to P1.1 and P8.4 and I have connected CO sensor output to P8.5.

Ouput with no connections to P1.1, P8.4 and P8.5

VeRef: 2046 A7 Value: 814 A6 Value: 609
VeRef: 2046 A7 Value: 670 A6 Value: 593
VeRef: 2046 A7 Value: 1532 A6 Value: 687
VeRef: 2046 A7 Value: 537 A6 Value: 570
VeRef: 2046 A7 Value: 939 A6 Value: 621
VeRef: 2046 A7 Value: 1397 A6 Value: 675
VeRef: 2046 A7 Value: 398 A6 Value: 554

Output with VeRef to P1.1, P8.4 and CO sensor to P8.5

VeRef: 2046 A7 Value: 2535 A6 Value: 519
VeRef: 2046 A7 Value: 2539 A6 Value: 616
VeRef: 2046 A7 Value: 2537 A6 Value: 688
VeRef: 2046 A7 Value: 2539 A6 Value: 500
VeRef: 2047 A7 Value: 2541 A6 Value: 664
VeRef: 2046 A7 Value: 2539 A6 Value: 649
VeRef: 2042 A7 Value: 2541 A6 Value: 510

Output with VRef to P1.1, Battery 1.5V to P8.4 and CO sensor to P8.5

VeRef: 2047 A7 Value: 1965 A6 Value: 527
VeRef: 2052 A7 Value: 1964 A6 Value: 679
VeRef: 2046 A7 Value: 1965 A6 Value: 558
VeRef: 2046 A7 Value: 1966 A6 Value: 535
VeRef: 2054 A7 Value: 1966 A6 Value: 671
VeRef: 2047 A7 Value: 1974 A6 Value: 525

Output with Battery 1.5V to P1.1, P8.4 and CO sensor to P8.5

VeRef: 2046 A7 Value: 1944 A6 Value: 544
VeRef: 2048 A7 Value: 1941 A6 Value: 598
VeRef: 2041 A7 Value: 1941 A6 Value: 590
VeRef: 2046 A7 Value: 1940 A6 Value: 560
VeRef: 2039 A7 Value: 1938 A6 Value: 623

Output with P1.1 and P8.4 to GND

VeRef: 2046 A7 Value: 0 A6 Value: 574
VeRef: 2046 A7 Value: 2 A6 Value: 462
VeRef: 2046 A7 Value: 0 A6 Value: 437
VeRef: 2046 A7 Value: 0 A6 Value: 535
VeRef: 2046 A7 Value: 0 A6 Value: 615
VeRef: 2049 A7 Value: 5 A6 Value: 552
VeRef: 2046 A7 Value: 0 A6 Value: 442

My questions are:

1. If the full scale resolution of 12bit ADC is 4096, why am I always reading around 2046 on VeRef? Even when it is not connected.

2. Why are values on the P8.4 not the same as P1.1 when both are connected to the same voltage?

3. Even when I connect the CO sensor pin to P1.1, the values are still showing 2046. It is the same with Battery of 1.5V

4. If I connect to both the VeRef and the ADCs to the same constant voltage source should I not get the same readings?

5. How can I calibrate P1.1 to 2048mV? What are calculation should use to get corrected reading on P8.4 and P8.5?

6.  What is the calculation to get the 2048mV on P8.4 when it is also connected to VeRef?

Thank you for your patience in advance.

Regards

Vijay

Hi Vijay,

I noticed you mentioned you are using REFOUT = 1. Could you please take a quick try to set it to 0?

When setting REFON and REFOUT to 1, P1.1 will output the internal reference out, and may have conflict if you connect some external voltage to it at the same time.

Meanwhile, according to your test data, I noticed that when you connect the 2.048V to P8.4, the result is around 2540, which indicates that the reference is around 3.3V (2.048/2540*4096 = 3.3V). Could you please also double check the ADC12VRSEL bits to see if it works as expected?

Also, I'm curious about how you get P1.1 result? Do you also read P1.1 from ADC channel A1? 

There are something wired in the P1.1 result, which I'm now have little clue with it.

Look forward to your update and also will let you know if I got something new.

Regards,

Harry

Harry

I have removed the switch from the launchpad and connected the P1.1 to the VeRef. There is nothing else wired to it. I am reading from the P1.1 just like I am reading from the P8.4 and P8.5. Is this wrong?

As per the design reference, the setting for VeRef and ADC is the same. So I assumed that P1.1 will act as ADC input for VeRef.

Below is the partial code

// REFCTL0 &= 0x00;

P8SEL1 |= 0x0F;
P8SEL0 |= 0x0F;

P1SEL1 |= 0x03;
P1SEL0 |= 0x03;
ADC12MCTL0 = ADC12VRSEL_4;

With VeRef connected P1.1 and P8.4 the out is

VeRef: 2047 A7 Value: 2529 A6 Value: 579
VeRef: 2047 A7 Value: 2539 A6 Value: 563
VeRef: 2039 A7 Value: 2533 A6 Value: 666
VeRef: 2046 A7 Value: 2540 A6 Value: 647

The yellow and orange wires are connected to VRef of the sensor board. I checked the VRef of sensor board with multi meter and it is exactly 2048 mV.

Hi Vijay,

Could you please try to set REFOUT = 0?

What I want to say is that you should not use P1.1 as reference and also read its "ADC conversion result" from A1. ADC uses reference to convert thus if you read out the reference value it should reach to full range (4095 here for 12-bit). But as you mentioned you have turned on REFOUT, the internal reference will also output from P1.1 (VRef+ function). So I suspect that there may be some conflict if you also put an external voltage at the same pin.

I have set the REFOUT=0. Even in my earlier code I have not set REFOUT to 1.

REFCTL0 &= 0x00;

P8SEL1 |= 0x0F;

P8SEL0 |= 0x0F;

P1SEL1 |= BIT1;
P1SEL0 |= BIT1;

ADC12CTL2 |= ADC12RES__12BIT;
ADC12MCTL0 |= ADC12VRSEL_4;

Below is the output with VeRef connected to P1.1, P8.4 and CO sensor to P8.5. I see no change. How can I know for sure that VeRef  is working without reading from it? Is there any workaround for this? I want to make sure that I get the same value on both P1.1 and P8.4 when I connect the external voltage source.

Regards

Vijay