TPS65950 (PMIC) Backup Battery Voltage giving unexpected readings

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Last Post 07 Apr 2017 06:54 PM by John Huang

TPS65950 (PMIC) Backup Battery Voltage giving unexpected readings

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John Huang

New Member
Posts:8

05 Apr 2017 09:10 PM
I'll state the obvious first, in case anyone needs some background. I am working with the Torpedo DM3730 SOM which has an integrated PMIC (Tps65950). According to the PMIC, we can stick a coin cell to the backup battery to keep the RTC running. Then the voltage of the coin cell can be read using MADC CH9. In the Linux BSP User Guide, it offers a script to do exactly just this. I have ran the script and I am able to read the ADC values for CH9 and the script also converts the ADC readings to milliVolts. Our coin cell has a measured voltage of 3V from a Voltmeter. The script actually reports that the ADC reading is 3.2 volts. This caused me to do further experiments. I modified the BB_SEL bits (Backup battery charging voltage) to 0x00 (2.5V). Then I re-ran the script. This time it reports back 2.5V. It tried the other settings for BB_SEL and it is starting to appear that the ADC reading in CH9 just simply reports back the Backup battery charging voltage rather than the ACTUAL coin cell voltage. Is this actually the expected behavior? Because our EE design team was actually expecting our software to report the ACTUALy coin cell voltage, not the charging voltage setting. BTW, the coin cell is directly connected to the backup battery ports. There is nothing (like a diode) in between or anything sinking sinking the charge current.

bradb

Basic Member
Posts:203

06 Apr 2017 03:31 PM
John, I ran the adc-script.txt script using the DM37x Linux Torpedo + WIFI development kit and received the following output. While running the script I viewed the voltage on the DVM and it was reading 2.405 volts. DM-37x# ./adc-script.txt GPCH9_LSB is 0x00 GPCH9_MSB is 0x89 GPCH12_LSB is 0x00 GPCH12_MSB is 0xb7 ADC9 value is 548 , 2410 millivolts Backup Battery voltage ADC12 value is 732 , 4293 millivolts Main Battery Voltage Thanks, Brad

John Huang

New Member
Posts:8

07 Apr 2017 03:55 PM
Hi Brad, Thanks for trying that out. So your script is report back about 2.5V. Is that the true voltage of the coin cell itself? Have you measured it with a volt meter? Also, is your BBSEL bits set to something other than 'b00 (2.5V charging voltage)? i2cset -f -y 1 0x4b 0x6d 0x1C When you change the BBSEL bits around, does that CH9 ADC value fluctuate much? If the Dev-kit is working as expected, then somehow it must be an electrical issue with our custom board. Thanks.

John Huang

New Member
Posts:8

07 Apr 2017 06:54 PM
Brad, No need to spend anymore time on this. I tried the same activity with the Dev Kit. I am consistently getting 2.1mV independent of the Battery Voltage setting. Well, it appears the coincell on the dev kit is rechargeable as it's voltage is going up over time. =) I route this issue back to my EE team. Thanks for that extra tidbit that I forgot to try myself.

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