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PGA970: SPI Communication and Primary Excitation

Joel Bommeli
Prodigy 10 points
Part Number: PGA970

Tool/software:

Hello.
I would like to ask if it´s possible to operate the PGA970 only with the SPI interface. This means without using the OWI interface or flashing a firmware via the SWD interface. In the datasheet the following steps are described to activate the SPI interface:

"7.3.1.14.2 Activating the SPI Interface

To activate SPI communication, perform the following steps in order:

1. Enable SPI by writing 1 to the SPI_EN bit in the DIG_IF_CTRL register.

2. Enable the digital interface by writing 1 to the IF_SEL bit in the MICRO_INTERFACE_CONTROL register.

3. If the SPI master is to communicate to the PGA970 device with the microprocessor in reset, reset the microprocessor by writing 1 to the MICRO_RESET bit in the MICRO_INTERFACE_CONTROL register. If the SPI master is to communicate with the PGA970 device while the microprocessor is running, then the COMBUF registers are used for communication."

  1. So now I have the following Questions:Can I write the DIG_IF_CTRL and the MICRO_INTERFACE_CONTROL Registers over SPI even if SPI isn't activated or have I to perform this via OWI or Firmware?
  2. If SPI then is activated is it possible to write Firmware to the device via SPI? Because I read in the datasheet, that Access to the whole Memory of the device is possible, if the microcontroller is in RESET state.
  3. Also some another About the PGA970: How big is the maximum Voltage the PGA can generate for the excitation of the primary coil. In the datasheet the DAC range is specified with 1.25 V. That means in Theory, the DAC can generate a Voltage between 0 and 1.25 V. But the differential Offset has to be adjusted between 0.82 V and 0.88 V. So I guess the maximum Amplitude I can reach is 1.25 V - 0.82 V = 0.43 V. So after the gain stage with maximum 1.67 V/V it's possible to reach an Amplitude of 0.43 V * 1.67 V/V = 0.71 Vp which results in 1.43 Vpp in differential mode. DIFF_VOCM_CTRL will define the DC Offset of this AC voltage.

So far, please tell me if if I wellunderstood the function of the waveform Generator.

Then, is it also possible to us the full DAC range for an Amplitude of 1.25 V / 2 = 0.625 Vp which should result in an Amplitude of 1.04 Vp after the gain stage?

I'm glad to to hear from you.

Joel

  • 0
    TI__Intellectual 2075 points

    Hi Joel, 

    Welcome to E2E! To answer your questions:

    1. SPI is enabled by default, so when you power up the device, you can use SPI to communicate with MICRO_INTERFACE_CONTROL. By default, the PGA970 is running (not in reset) so you cannot communicate with all the registers - you must put the device in reset before you can communicate with all the registers. You can put the device in reset by writing 0x03 to MICRO_INTERFACE_CONTROL. 

    2. Yes, you can use SPI to write firmware to PGA970. You can write to FRAM or DEVRAM - they are just in different memory locations. As you note, you must put PGA970 in reset before you can communicate with those registers. 

    3. Yes, your understanding of the waveform generator is correct. 

    Joel Bommeli said:
    Then, is it also possible to us the full DAC range for an Amplitude of 1.25 V / 2 = 0.625 Vp which should result in an Amplitude of 1.04 Vp after the gain stage?

    I don't think so, that would violate the differential offset. You can use an external amplifier if you need a larger amplitude for your excitation signal. 

    Thanks,

    Maggie