pmbus: dealing with unsigned mantissa in linear reading

pmbus: dealing with unsigned mantissa in linear reading

[Chris Packham]

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?

Thanks,
Chris

Re: pmbus: dealing with unsigned mantissa in linear reading

[Guenter Roeck]

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.

Not sure what you mean with "standard linear encoding can't support values
>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. 

Guenter

Re: pmbus: dealing with unsigned mantissa in linear reading

[Chris Packham]

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