PAC1932SLASH3SLASH4 datasheet(25/62 Pages) MICROCHIP

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DS20005850C-page 25

PAC1932/3/4

4.9.1

ADDITIONAL ACCUMULATOR

INFORMATION

The math for the Power calculation and accumulation

inside the chip is always done in two's complement

math, no matter what the user sets the output registers

to show. VBUS and VSENSE are 17-bit two's

complement (signed) numbers internally. VPOWER is

the product of VSENSE multiplied by the 14 MSBs of

VBUS, and this is a 31 bit two's complement result

(signed) internally. In some cases this results in a

Power result that is not identical to the product of the

VBUS results register multiplied by the VSENSE register.

However, the Power result from the Power results

cases, as explained below.

If VSENSE and VBUS are both programmed to be

unsigned (unipolar) in register NEG_PWR (Address

1Dh), 16b without sign are exported to VBUS and

VSENSE results registers.

If VBUS is programmed to be signed (bipolar) in

NEG_PWR

(Address

1Dh),

the

corresponding data is truncated to 16-bit two's

complement (signed) for the readable results register.

If VSENSE is programmed to be signed (bipolar) in

results register value is truncated to 16-bit two's

complement (signed), but the power calculation uses

17-bit two's complement (signed). Therefore, a

mismatch is possible between an externally calculated

power value (VBUS times VSENSE) and the actual power

value calculated internally to the chip. The internally

calculated (and accumulated) value is more accurate

than the externally calculated value in every case.

The continuous power integration periods (also called

the energy accumulation period) can range from ~1ms

to many hours, depending on the number of samples

per second selected via SMBus. The number of

samples is limited by the size of the Accumulator Count

about 273 minutes at 1024 samples/second, or 582

hours at eight samples/second. This Accumulator

Count can overflow, and it will not reset when it

overflows.

When the accumulation registers reach their maximum

value, this is called accumulator overflow. The

accumulator outputs remain at their maximum value;

they do not roll over. The user can calculate the

worst-case time to roll over and read them at or before

that time or use the built in Alert functions to detect

rollover and read them at that time.

Worst-case accumulator overflow time can be

calculated assuming that every measurement that is

accumulated is a full-scale number. Since the power

numbers are 28 bits, and the accumulator is 48 bits, 220

samples can be accumulated before overflow if they

are all full-scale values. For most applications, they will

not all be full-scale numbers; this is especially true if

VBUS is not 32V. If VBUS is a lower number, the

maximum number of full-scale samples that can be

accumulated is scaled by 32V/VBUS. This limitation can

limit the accumulation period before overflow to 17

minutes at 1024 samples/second, or 36 hours at eight

samples/second, if most values are near full-scale. The

Accumulator Count limit described above will still limit

the total number of samples to 224.

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