LM629: IC generates PWM sign/mag at power up

Hi Jamie,

If MCU pin is HIGH all the time during its POR I think one of the solutions might be to

use an inverter gate between MCU and Reset pin of LM629.

Regards,

Grzegorz

Hi Grzegorz,

Thanks for your reply. If you mean the MCU pin connected to the RST pin the LM629 then I can confirmed this pin was LOW during POR. This is why I added the pull down resistor as described above to ensure the RST pin was not floating during POR (pin from the MCU is HI-Z by default).

Regards,

Jamie

Hi Jamie,

Thanks for your answer.

I meant that RST pin, but if it is LOW during POR the solution with an inverter will not work.

I guess keeping low LM629 RST pin is not enough to perform its reset. It may need clock signal

as well. In case of LM629 itself probably I will not be much of help.

If MCU pin connected to RST pin of LM629 is kept low during POR then it might be possible to

use it to gate PWM MAG signal from LM629 to motor driver.

Just use 2-input AND gate, connect output to PWM input of motor driver, one input connect

to LM629 PWM MAG and the second one to MCU pin connected to LM629 RST pin

(there still may be some single PWM impulses passed during MCU performing reset

of LM629) or better to other MCU pin if there is any available.

Regards,

Grzegorz

Hi Grzegorz,

Yes I think without additional circuitry it won't be possible to resolve this issue as it is. Regarding the RESET of LM629 the datasheet states:

Pin 27 (15), Reset (RST) Input: Active-low, positive-edge triggered, resets the LM628 to the internal conditions
shown below. Note that the reset pulse must be logic low for a minimum of 8 clock periods. Reset does the
following:
1. Filter coefficient and trajectory parameters are zeroed.
2. Sets position error threshold to maximum value (7FFF hex), and effectively executes command LPEI.
3. The SBPA/SBPR interrupt is masked (disabled).
4. The five other interrupts are unmasked (enabled).
5. Initializes current position to zero, or “home” position.
6. Sets derivative sampling interval to 2048/fCLK or 256 μs for an 8.0 MHz clock.
7. DAC port outputs 800 hex to “zero” a 12-bit DAC and then reverts to 80 hex to “zero” an 8-bit DAC.

So a simple pulldown would not fix the problem. I've also found in our design the the MCU supervisor IC (MAX709RESA) means there is approx. 150ms delay between LM629 VCC becoming stable and the MCU executing the code to perform the RESET of the LM629.

I think for future reference for anyone reading this is worth knowing that the LM629 does not disable motor output by default. I believe that the LM629 won't do anything if the CLOCK is missing, so using a switch to enable/disable the CLOCK might be simple solution. Or power switching the LM629 itself.

Regards,

Jamie

Thanks Grzegorz for the comments.  

Jamie,

This does seem like an unfortunate operation of the device.  Have you verified that gating the CLOCK solves the problem?  If so, please update for benefit to others on the forum. 

Regards,

Ryan

Hi Ryan,

I don't have a circuit I can use to test it operationally. I do have another board where the 8MHz oscillator has been removed and I can confirm the LM629 does not output anything (or operate at all).

Regards,

Jamie