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DRV8308: Is there a way to read direction or measured speed via SPI

Dale Ulan1
Prodigy 20 points
Part Number: DRV8308

I have workarounds for most of this but it would save me a bunch of wiring to be able to read the measured speed and the direction (or perhaps direction error) bits via SPI. Is there a way to do this? Also, the one second restart delay is a bit inconvenient though if the DRV8308 doesn't glitch and disable I can mostly work around it. I have a high inertia load (a tape machine using two inch tape, a 10.5 inch reel on each motor). I have some suggestions on the speed controller if anyone is planning another revision of the part, because I could not use it for my application. I have to do it externally and use SPI register write to make it work. What would be nicer is a setup where the speed controller output can be limited to, say, +/- 30 or 50 counts of the duty cycle, and a separate open-loop duty cycle written in as an adder. The range of the built-in controller doesn't work very well with a high inertia load.

  • 0
    TI__Guru 65200 points

    Dale,

    Thanks for posting on the MD forum!

    Unfortunately the speed reading is only available via inference using the FGOUT pin, not via SPI.

    We thank you for your recommendations but we do not plan to update the device.

    If it's easier for you we can recommend some new gate drivers which you can use to implement your own speed loop and algorithm.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 20 points

    Well, it’s easier to work around it with the DRV8308, the cost to do the work around is still much lower for my volumes than it would be to develop reliable software for sinusoidal commutation with hall sensors, etc. It just would be a lot easier if the IC had a way to read speed and also actual motor direction. There are a few other small things I would do differently for an IC like this one (allowing the PWM generator to modulate braking, for example). A driver and microcontroller combination in a package similar in size to this one, and that could implement the functionality of this with a few modifications could be interesting, otherwise there are a few of the DRV family that look ok, though the DRV8308 is quite unique and offers a very compact and ready to go solution.

    The workarounds are simple, reading direction just uses a SOT23 D flip flop (which TI makes) off the hall sensors, tach comes off of FG out. It does take two extra wires, but that’s not terrible. I am using the LTC6820 to run a remote SPI interface from the system controller to each motor, mostly for noise isolation, but the whole system is positive ground which makes logic level interfacing something of a challenge.