On 16/06/21 1:46 pm, Guenter Roeck wrote: > On Tue, Jun 15, 2021 at 11:34:53PM +0000, Chris Packham wrote: >> Hi Guenter, >> >> I've had a report from someone testing the BPA-RS600. >> >> When they have the input voltage set to >= 256V (apparently the PSU can >> handle input up to 264V). The Vin reading goes negative. >> >> CMD 0x88, VAL=0xf3f8, Vin=254 >> CMD 0x88, VAL=0xf3fc, Vin=255 >> CMD 0x88, VAL=0xf400, Vin=-256 >> >> Looking at pmbus_reg2data_linear() the mantissa is cast to s16 which is >> correct according to section 7.1 of the PMBUS spec which says that the >> mantissa is an 11-bit two's complement value. >> >> It seems that the BPA-RS600 is using a non-standard format for the >> mantissa (probably because the range of the standard linear encoding >> can't support values >255V). Does the pmbus infrastructure provide a way >> for me to define a custom format/conversion for a given sensor? > Essentially you'll have to implement a custom read word function and > convert the value reported by the power supply to a valid linear11 > value. In this case, you may have to shift the reported value by 1 bit > and increase the exponent by one. OK I'll look at doing that. I already have bpa_rs600_read_word_data() so I can hook in there. > Not sure what you mean with "standard linear encoding can't support values Yeah I figured that out after I sent the email. There are a few ways of representing >256V in a standard conforming way. >> 255V". Why not ? That is what the exponent is for, after all. It rather > seems to me that the PS vendor decided to violate the standard to get > another bit of accuracy. Indeed that seems to be the case. > Guenter
