Here we will describe more about how the J1850 circuitry works. To begin with, to change between the levels required for PWM and VPW variants of J1850, use this signal:
This signal is connected to Pin 123 (PB8) of the SAM3X.
Make this pin HIGH for PWM and LOW for VPW
This is an example of some code that will both turn on power to J1850 circuit AND set level for either PWM or VPW:
Testing hardware VPW interface:
Let's start with receiving J1850 VPW (Variable pulse width).
Here is o-cope shot of 2 signals. Top is
J1850_VPW_RX, botton is
Testing hardware J1850 PWM interface: