About brentbop

I'm trying to resolve a persistent issue I'm having with a BNO055 IMU on a custom PCBA. I'm communicating via I2C and am experiencing different behavior between device reset (using "Power on Reset" reset pin) with and without power cycling. In situations where I reprogram my MCU and start directly into my application code without power cycling, the IMU will respond with NAKs to any and all read/write messages. If I power-cycle the system, it works correctly. In both scenarios I'm pulling the reset pin low for ~25ms, pulling the reset pin high, and then waiting for ~725ms before initiating I2C communication. All other signals (COM1, COM2, PS0, PS1, nBOOT_LOAD_PIN) are connected to VDD or GND directly or through a 10kΩ resistor. A partial schematic is attached. The firmware is heavily based upon the BNO055 XPlained Pro example code and is being run on a SAMD21G18A. Can you help me to understand the differences in behavior between device reset for the BNO055 from an 'unknown' state and from a power-on state? Do I need to set the SDA/SCL lines to a certain level before/during/after Power on Reset? Attached are logic analyzer screenshots of the system failing without power cycle (BNO055_noPowerCycle.png) and working with power cycle (BNO055_withPowerCycle.png), as well as a partial schematic (BNO055_sch.png). ... View more