BQ76920: CC issue: MSB (sign bit) always 0 in cc continuous mode

mirko bagnoli

Part Number: BQ76920

I'm able to read correctly all registers (VCs - TS - BAT-HI and BAT-Low and all other registers) but CC seems not correct.

when I read CC in continuous mode, positive values are ok (charging) and show correct number of mV on shunt,

but negative values (discharging) always first bit (MSB of CC_HIGH) is 0... so values go from 32767 downwards... (although the difference from 32767 - read value is a correct measure of the mV on the shunt)

the 2's complement should use the MSB as sign I think... am I wrong?

what am I doing wrong?

(I am using as host an atmega328p)

over 6 years ago

0 Matt Sunna over 6 years ago

TI__Guru* 81034 points

Hi Mirko,

You are correct - the MSB should be 1 for negative values. Did you write the CC_CFG register to 0x19?

Matt

0 mirko bagnoli over 6 years ago in reply to Matt Sunna

Prodigy 50 points

yes I put reg 0x0B to 0x19 :-)

0 Matt Sunna over 6 years ago in reply to mirko bagnoli

TI__Guru* 81034 points

That's good. Are you able to view the waveform directly on a scope to verify that the MSB is really zero? The MSB should be high for negative values.

0 mirko bagnoli over 6 years ago in reply to Matt Sunna

Prodigy 50 points

will try with a scope... but have it not directly available since I'm actually working only on the firmware... in any case other registers that have first bit set (like for example SYS_STAT (the CC_Ready bit)) comes over the i2c correctly... do you think there could be any timing errors? or what are you thinking about? many thanks!!!

0 mirko bagnoli over 6 years ago in reply to mirko bagnoli

Prodigy 50 points

I tried a bit around and noticed that if I request only the one byte CC_High and then immediately I request the 2 bytes of the CC (with auto increment), then I will have in my variable all 16 bits... also the previously missing MSB... strange... what could be the cause in your opinion?

0 Matt Sunna over 6 years ago in reply to mirko bagnoli

TI__Guru* 81034 points

Hi Mirko,

I'm not sure, but the recommendation is to read both bytes together because otherwise the value may change between capturing the high and low bytes. Unfortunately, I don't know how to explain the missing MSB you observed when reading the bytes separately.

Matt

0 mirko bagnoli over 6 years ago in reply to Matt Sunna

Prodigy 50 points

in fact I read the 2 bytes together in one time. but if I do only this the first bit is missing. but If before reading the 2 bytes together, I read only the CC-High byte (without storing or using it) and after I read the 2 bytes... the reading is complete, and also the MSB is there... strange... did it never happen to you? Or someone that is reading here? Thank you for any opinion!

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BQ76920: CC issue: MSB (sign bit) always 0 in cc continuous mode