PWM Interrupts / Interrupt Priority Levels

Zephyr 3.2.99 - nRF Connect 2.2.0

We have a 4 digit 7 segment display that we use on one of our products. Instead of using something like timers to keep the display updated we designed it to use PWM interrupts instead. So every time an interrupt happens we increment the current digit and set the PWM pulse cycles / brightness. The PWM is inverted and it's set to use a 4ms period. So, each segment can have a min brightness of 4ms - 100us (3,999,900ns) and a max brightness of 160us (160,000ns). We never use full brightness (0ns) or no brightness (4,000,000ns) because the interrupts would get disabled and cause the display to no longer work. This has been working great, but we noticed there was a "bleed" on the display (some of the segments are partially lit when they shouldn't be). After looking into it we found that the interrupt callback timing was not consistent. The callback gets fired usually around 130us to a little over 200us (normally around 150us). This is causing the digit switch to be delayed, which partially lights a segment up. This all goes back to the max brightness; the lower the value the more it bleeds and the higher the value the more stable it is (but at the sacrifice of a dimmer display). Here is a visual example of what we are seeing:

"Set Digit" (the top reading) happens as soon as the interrupt callback is fired. The "Display Seg A" is a wire hooked to the capacitor that the segment A LED uses.

Here is what it should look like and has no bleeding (you can see it fires before the period starts giving it enough time to switch digits properly):

I guess I have three questions.

1) Is there any way to make the callback a little more consistent, or can the variable 130us - 200us be expected? I know there's always going to be variable timings, but it would be nice to minimize the range if possible. My goal is to find the optimal max brightness value that causes the least bleeding.

2) Is there any way to shorten the time it takes for the callback to be fired? The quicker it gets called the brighter we can make the display. I don't think it will make a drastic difference, but every little bit helps.

3) One thing we tried was to increase the priority of PWM1 and PWM2. By default everything has a priority of 1 (NRF_DEFAULT_IRQ_PRIORITY), so we change this to 3 and made the PWM1 and PWM2 have a priority of 2. This didn't help. However, it does bring up a question about the interrupt priorities. According to the documentation (https://infocenter.nordicsemi.com/index.jsp?topic=%2Fsds_s140%2FSDS%2Fs1xx%2Fprocessor_avail_interrupt_latency%2Fexception_mgmt_sd.html) it states that priority level 1 is reserved by the SoftDevice. So, is it OK to use priority level 1 (like NRF_DEFAULT_IRQ_PRIORITY) or should we stick to using 2 & 3? We want the display to have the highest priority over the other devices.

Here is a quick mock-up of something similar to what we're doing (see configure_pwm and pwm_callback):

Appreciate any help.