PWM stability issues (resetting)

Question

I'm using: nrf52832, SDK 15.0, SoftDevice S132 v6.0. 
 I'm trying to control a motor using an H-Bridge. If I simply do a nrf_gpio_pin_write to set forward/reverse to 1/0 or vice versa, there is no issue. I can change motor directions quickly. 
 However, I want to control the motor speed as well. When I use PWM outputs to control forward/reverse, the system will crash/reset when I try to change the motor direction. I can add a delay in the code where, between changing direction, I set both forward and reverse to 0 and wait 600ms. When I add this delay, everything works, but I want to minimize or remove this delay (400ms in unstable, 500ms is mostly stable, 600ms is stable). 
 I've tried two different h-bridge boards with different chips (L9110S with simply forward/reverse and TB6612FNG with 0/1 on forward/reverse and a single PWM input for speed) and have seen the same issue. 
 Why is the PWM output so much more sensitive to noise/back-emf? And can I do something to avoid it? Or should I be using something other than PWM? (I've tried both the PWM driver and the low power PWM code). 
 A few notes: I've tried capacitors across the motor. The design requires a single shared power source (batteries). I'm regulating it to 3.3v. 
 UPDATE: I decided to do my own PWM type code without using the library and had the same problem. I then realized I've been using certain pins for PWM and when I could successfully reverse the motor before, I was using two different pins. After some testing [simply using nrf_gpio_pin_write] pins P0.11 and P0.14 are working for forward/reverse (and combinations using P0.12 and P0.13 are not). I am using a board I bought online, so there's a possibility that it's a factor. 
 UPDATE 2018-07-24: For testing, I'm just having it alternate forward/reverse every 2 seconds and it will sometimes reset. It might run for 10 seconds, it might run for more than a minute. I thought RTT might give me some insight but I don't see any error info--I do see debug messages I added. Any thoughts on why this is happening are appreciated. 
 Also, after the chip resets, I've read NRF_POWER->RESETREAS, and I get a value of 0.

Einar Thorsrud · Accepted Answer

This is probably due to difference in internal resistance of the GPIO pins which can vary a bit (also between chips). 
 I suggest the following approach to fix this issue: 
 
 Determine the maximum current consumption of the motor (typically when it starts). 
 If the maximum current consumption is lower than the IOH,HDH , then you should be OK by just configuring the pin as high drive. Remember to use the minimum value (6 mA) so that the value is valid for any nRF52832 device, as there can be significant variation. 
 If the current consumption of the motor is higher, then you need to use a form of buffer so that you can supply the motor separately and not via the nRF52.

Einar Thorsrud · Answer

I see. Since you only see this with the motor connected, then I think we can say with high certainty that the reset is actually due to a BOR, and that you need to revise your HW to prevent the supply voltage from ever falling below 1.7 V. You would need to get hold of an oscilloscope or similar to say for sure, though. (A multimeter is not sufficient, as the voltage drop might be very short in time.)

PWM stability issues (resetting)

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