20. Write C code that creates an output on PA3 which is high for 1 second and low for 3 seconds and repeats. Use SysTick interrupts to time the events. a) Write the GPIO PA3 & SysTick-Init function/procedure. b) Write the SysTick ISR. 20. Write C code that creates an output on PA3 which is high for 1 second and low for 3 seconds and repeats. Use SysTick interrupts to time the events. a) Write the GPIO PA3 & SysTick-Init function/procedure. b) Write the SysTick ISR