We are experiencing strange behaviour regarding ship mode on BQ25121AYFPT. When we plug the battery into the system, it starts in ship mode after this when /MR presses the Vsys Rise. The problem is when we are again trying to enter into ship mode by:
it does not respond both ways, while the /CD pin is pulled high to the Vsys pin of the charger IC.
We are using a 300mAh LiPo pouch battery. Vin(charger) is disconnected. Battery voltage at that time was 3.86V.
We tried to enter ship mode using multiple methods. Below are the findings after each method -
Case 1: When BQ25121A is Controlled by STM32 MCU -
1.1: Through I2C Command
the voltage would start rising in pulses a few milliseconds after the ship mode command was executed. On the PMID pin, the pulse continues rising until the PMID reaches full voltage. The pulse shape and timing look similar on these pins. I have attached two screenshots of an oscilloscope for reference. In the first image, Channel:1(Yellow) represents the /MR pin and Channel:2(Green) represents the /RESET pin of BQ25121. In the Second image, Vsys is plotted. This issue causes a perpetual state of on-off cycles as we can see on an oscilloscope image. Resistor Settings were as follows when it starts oscillating:-
Register[0] uint8_t 100011
Register[1] uint8_t 1000000
Register[2] uint8_t 10001000
Register[3] uint8_t 11101010
Register[4] uint8_t 110
Register[5] uint8_t 1111000
Register[6] uint8_t 11111110
Register[7] uint8_t 11011000
Register[8] uint8_t 11000000
Register[9] uint8_t 111111
Register[10] uint8_t 1111000
Register[11] uint8_t 0
1.2: Through /MR Push Button
There was the same observance on PMID, /MR, and Vsys pins when looking at the oscilloscope.
Case 2: When the Vsys line of BQ25121A is disconnected from STM32 MCU, /CD disconnected from MCU and pulled high to Vsys through 10K
When we plug the battery into the system, it starts in ship mode after this when /MR presses the Vsys Rise. but it does not enter into ship mode when /MR is pressed. We are assuming that all the resistor settings would become the default in this method and if the IC successfully goes into ship mode, we can benchmark at least the charging IC.
Yes, PMID falls as expected and rises back in 6 mS repeatedly. In our application, we are using it with an STM32WB55CGU6 microcontroller. The nRST pin of the MCU is directly connected to the /MR pin of the charging IC. We also identified that this is the reason behind the repeated rise and fall of PMID but Before making adjustments in our new design to address this issue, we wanted to evaluate the ship mode manually within the existing circuit.
To do this, we disconnected the MCU's power line from Vsys and proceeded to manually benchmark the BQ25121A by asserting the /MR button (Case: 2) as discussed earlier.
After disconnecting the MCU power and Vsys, upon plugging in the battery, the system initiated in ship mode as expected. Pressing the /MR button caused Vsys to rise, effectively exiting ship mode. However, when attempting to re-enter ship mode by manually pressing /MR again, the expected ship mode entry did not occur. We are assuming that all the resistor settings of BQ25121A would be default in this method.
This evaluation was aimed at understanding the standalone behaviour of ship mode in our existing setup before making alterations to the new design.
I am attaching three screenshots, in which /MR probe is Yellow, Vsys probe is Blue and Vbat probe is Green.
1. BatPlugIN: when system initiated in ship mode. 
2. ExitingShipMode: when Pressing the /MR button caused Vsys to rise.
3. NotEnteringShipMode: when expected ship mode entry did not occur.
Any guidance would be greatly appreciated.
