We are using K60 microcontroller and MQX RTOS in our product and recently we faced a problem related to the RTC module. There are two cases and questions, explained below:
1) VBAT is powered by a coin cell battery: In this case the VBAT remains powered even if system is powered down. When we power the board, we see that the microcontroller gains control in around 32ms. We checked this by toggling a GPIO pin in the main function. The controller starts toggling the pin ~32ms after the reset pin goes high. What could be the reason for controller to take this much time? Could it be MQX? I want to know the average boot up time of K60 with basic MQX code running.
2) VBAT is powered by 3.3V used to power the K60: In this case NO battery is used. So every time the boards boots up, the K60 and its RTC module get their power supply from the same 3.3V source. I found from the manual that in this case the RTC module's POR will come in picture each time the boards is powered down and powered up. But the manual also mentions that this POR is only for RTC module and it does not affect system reset. However, when we did the experiment mentioned in above case, we observed that it takes around 532ms for the GPIO to toggle! That's 500ms additional delay. I am not able to understand the reason, but suspect that there could be some relation between MQX and RTC. What can be done to verify and solve this issue?
I have attached the waveforms of both cases.
Orange: 3.3V power supply
Purple: Reset pin
Green: GPIO pin
Please let me know if you need more details.