We are currently verifying that our design meets the power up sequence per the reference manual. We are bypassing the internal DC/DC and all power rails come up in the correct sequence. However, we have an intermittent issue that occurs sometimes during a brief power cycle. The root cause appears to be the RT1176 "PMIC_ON_REQ" going low, about 5 mS after "VDD_SNVS_IN" goes high.
I can't seem to find anywhere in the reference manual that explains the cases when "PMIC_ON_REQ" would go low. There is one mention of when the "DCDC_IN" goes below 2.6V that will cause “PMIC_ON_REQ to go low. However, "PMIC_ON_REQ" still goes low and stays low even though “VDD_SNVS_IN” and “DCDC_IN” have remained stable throughout powerup.
I found that I wasn't handling the RT1176 POR_B (pin T10) according to the MCU’s power sequencing specifications. The MCU reference manual states that the POR_B must remain low until all the power rails are stable. In my case, I inadvertently had the input to POR_B indirectly dependent on the PMIC_ON_REQ. The details aren't important but I basically had a sort of a circular logic going on.
In other words, POR_B was glitching during the power up and was telling the MCU to reset before it had a chance to power up. I also slowed down my overall power sequencing timing, allowing some margin for each of the power rails to come up and waited awhile before pulling POR_B high. Here is a plot of my sequencing that is working now ("MCU_RESET is the name I gave POR_B):
Did you ever figure this out? I have the exact same issue with the PMIC_ON_REQ going low after ~5 ms.
Could you expound on your comment about the relationship between ON/OFF and PMIC_REQ_ON?
I have hardware that more consistently shows PMIC_REQ_ON going low after 5mS. How is the ON/OFF signal linked to the PMIC_REQ_ON dropping low after 5mS after initial power-up? I haven't seen any specific mention of this behavior related to power sequencing. The consistency of the 5mS seems like the outcome of some logic rather than a transient voltage/current blip on the ON/OFF or PMIC_REQ_ON.
I've included two oscilloscope screen captures of our power sequence which also includes the ONOFF signal. In one capture, I have tied ONOFF to ground via a 6.6K ohm, and the other screen capture is with the ONOFF left unconnected for the MCU internal pullup control.
Could you clarify where the series resistor should be placed: ON/OFF or PMIC_REQ_ON?
Hi @mckaylund ,
ON/OFF is used to switch PMIC_REQ_ON to control power modes. A 4.7-kΩ to 10-kΩ series resistor can be used when the current drain is critical.
Regards,
Jing