get PWM values via i2c
On Raspberry Pi (any model) you have a number of GPIO inputs. You may want to program them for rising and falling edge detection, marking the timestamp on each of the events and calculating the PWM programmatically. This however requires Linux kernel driver programming skills as userspace interface to GPIO may be too slow to provide an adequate response.

Alternatively, you could use Raspberry Pi's ADC input connected to your incoming PWM signal via a low-pass RC filter.

You may also use an external I2C ADC with a RC low-pass filter or you may craft your own digital I2C PWM reader using an additional STM32F030 chip and it's general purpose timer in Input Capture mode.

You may want to review the answers to this question at Raspberry Pi StackExchange: https://raspberrypi.stackexchange.com/questions/9787/pwm-input-in-raspberry-pi