TMS320F28377S: Launchpad TMS320F28377S Buffered DAC A Clipping

Daniel Kakaley

Part Number: TMS320F28377S
Other Parts Discussed in Thread: CONTROLSUITE, C2000WARE

Hi,

I am using a 28377S Launchpad to get code working for the Buffered DAC A Output pin on the Launchpad / Processor. Right now I am scoping Launchpad pin 27 and I am seeing some weird behavior with clipping of the DAC output.

First of all, I have working code which changes the DACVALA field in the DacaRegs Register (checked by looking at the register values through the JTAG header on the launchpad). I am cycling between 20 and 4020 bits on the register with a 1/2 Hz sine wave, which should correspond to ~0 to 2.9 Volts on the Launchpad. However, I get the following results on the scope. The lower end of the Sine wave is clearly visible, but the upper end (above 700mV is saturating). I am literally using nothing except for a 28377S Launchpad.

First, is there some reason I am not considering that the DAC A output would be saturating on the processor?

Second, is there an issue with the launchpad 28377S hardware?

I have tried switching between the VREFHI at 3V on the Launchpad and an external VDAC reference, it does not change the results of this. Any ideas?

over 8 years ago

0 Frank Ankapong over 8 years ago

TI__Mastermind 28275 points

Hi Daniel,

A few questions to better understand what is going on:
1. What code are you using to output that sine. Is this an example from ControlSUITE/C2000Ware or your own example?
2. What happens if you just output high code like 4095? This is to make sure the DAC is getting a correct reference voltage.

0 Daniel Kakaley over 8 years ago in reply to Frank Ankapong

Prodigy 225 points

1) It is essentially my own example, I am using a Simulink Model to generate the C code for this. The sine wave is being generated "in software" as expected as a uint16. I am wondering if it could be the 16 bit value being inserted improperly into the 12bit DACVALA field.

2) Outputting a high code like 4095 just outputs ~700mV signal according to the scope. I also measured it with a fluke and got 0.670 V DC.

0 Frank Ankapong over 8 years ago in reply to Daniel Kakaley

TI__Mastermind 28275 points

Looks like a diode drop where the DAC isn't getting a reference voltage or supply power. To completely rule out any software issues, can you run the buffdac_enable example code in controlSUITE/C2000Ware, output a high code and see if the same still happens.

0 Daniel Kakaley over 8 years ago in reply to Frank Ankapong

Prodigy 225 points

Same thing happens... 0.67V on pin 27 of the launchpad while running the buffdac_enable code in controlsuite

0 Frank Ankapong over 8 years ago in reply to Daniel Kakaley

TI__Mastermind 28275 points

OK. Have you done any modifications to the launchpad and do you have JP1 and JP2 jumpered?

0 Daniel Kakaley over 8 years ago in reply to Frank Ankapong

Prodigy 225 points

No modifications. JP1 and JP2 are jumpered, and I did measure 3V on the voltage reference for the ADC... so that appears to be feeding the MCU with the right voltage.

0 Frank Ankapong over 8 years ago in reply to Daniel Kakaley

TI__Mastermind 28275 points

Ok. Starting to look like you might have a bad device or board. Try a few more experiments:

1. Try DACB and DACC.

2. The DAC uses the same reference as the ADC so you can run the "adc_soc_software" example, put a mid-voltage on the expected pins and verify if the results make sense.

0 Daniel Kakaley over 8 years ago in reply to Frank Ankapong

Prodigy 225 points

Ok. Looks like I found the issue. I was not using a high impedance scope probe which was loading the DAC.

I had two launchpads with DAC issues, one seems to be a dead DAC that is 0V no matter what, the other does this 700mV thing, but that is fixed once I use the scope probe, so I'm good to go now.

0 Frank Ankapong over 8 years ago in reply to Daniel Kakaley

TI__Mastermind 28275 points

Glad the issue is resolved. Let us know if you ran into any more issues.

C2000™︎ microcontrollers

C2000 microcontrollers forum

TMS320F28377S: Launchpad TMS320F28377S Buffered DAC A Clipping