• TI Thinks Resolved

LMH6517: SPI problem

Part Number: LMH6517

Hello!

I am trying to program the gain of the LMH6517 via SPI using a beagleboard, but that does not seem to work. I already checked the communication lines on the oscilloscope and everything seem fine, so I would like to know if anyone can help identify what the problem is.There is some fixed gain there because I get an amplified signal out of it, but it does not change. The latch pin should not matter when spi is in use so I wonder what could be happening.

This would be the python code:

from Adafruit_BBIO.SPI import SPI

spi = SPI(1,0) #configure device and bus at beagleboard

spi.msh = 10500000 #clock
spi.bpw = 16 #bits per word

spi.open(1,0) #opens it

spi.xfer2([128,0]) #transmits data

Thank you,

JPL

  • In reply to JPL:

    It was just a probe issue here, the signals are correct... I am back to the start.

    Since I get the signal transfer if I use just the beagleboard and connect MISO/MOSI together, the problem would not have been there I guess.

    On page 13 here the SPI settings are given, I tried changing the mode before and it didn't help. Just to be on the safe side, it should be mode 3, am I right?

  • Hi JPL,

    Based on your images, it looks like you aren't addressing the SPI properly. It looks like your MOSI (SDI) signal is just kept high for the first 8 bits. In order to write the SPI for the LMH6517 the first 8 bits need to be set according to table 3 in the datasheet. The first 8 bits are the command bits that tell the SPI if it is a read/write, and what address to write to. By default the addresses are 0h for channel A and 1h for channel B. So to write the channels, your first 8 bits need to be...

    00000000 to write channel A
    00000001 to write channel B

    Hope this helps to explain.

    Regards,

    Jacob Freet 
    High Speed Amplifiers

  • In reply to Jacob%20Freet:

    Hi Jacob Freet,

    Even when sending like that the chip does not respond, I get the same constant gain when sending these commands for example:

    I redesigned my board to use the LMH6517 on pulse mode, I don't know what is happening with the SPI. I tried programming a different PGA here that I also designed a circuit using SPI and for the other one my code worked, so I guess the LMH6517 requires something else that I was not able to find out.

    Thank you,

    JPL

  • In reply to JPL:

    Hi JPL,

    I'm glad you were able to get it working with the pulsed mode. If you ever need to switch to the SPI please contact us and I'm sure we can get it to work successfully.

    Regards,

    Jacob Freet 
    High Speed Amplifiers