Posted on June 08, 2018 at 12:48Writing 0x8000 or 0x0000 into GPIOA->ODR in most cases shouldn't cause the part to reset. That it resets in your situation suggests some other external connectivity, or paths, that result in the part resetting. For example one of the pins shorting with VDD, or causing some other chip to assert NRST causing a global reset. In the case of a GSM modem perhaps the massive draw of current beyond the capacity of the regulators/supply to sustain an operating voltage, causing a brown-out where the POR circuit thresholds.
Look at the design and implementation of the board at a higher level.
Try different bit patterns on GPIOA->ODR to identify the specific pin(s) being the issue, ie write 0xFFFE, 0xFFFD, 0xFFFB, etc.
