Conversion from direct format to real-world dimensions of current, voltage, and power is accomplished by determining the appropriate coefficients as described in section 7.2.1 of the PMBus Power System Management Protocol Specification 1.3 Part II. According to this specification, the host system converts the received values using Equation 3 into a reading of volts, amperes, watts, or other such units.

where

• X = the calculated real-world value (volts, amps, watts, and so forth)
• m = the slope coefficient
• Y = a 2-byte, two's complement integer received from the device
• b = the offset, which is a 2-byte, two's complement integer
• R = the exponent, which is a 1-byte, two's complement integer
• R is only necessary in systems where m is required to be an integer (for example, where m can be stored in a register of an integrated circuit) and R must only be large enough to yield the desired accuracy

The values for m and R (listed in Table 6-1) must be calculated for current and power measurements based off the selected value of the Current_LSB. For example, assume a Current_LSB of 0.75mA/bit is selected for a given application. The value for m is calculated by inverting the LSB value (for this case, m = 1 / 0.00075 = 1333.333). Moving the decimal point so the value of m is maximized and remains within the required range of –32768 to 32767 is preferable because this value of m is relatively small and contains decimal information. Moving the decimal point one place to the right results in a final m value of 13333 with an R value of –1 resulting from the shift in decimal location. Moving the decimal point to maximize the value of m is critical to minimize rounding errors. The m coefficient for power can be calculated by applying 1 / (25 × Current_LSB). For this example, the value for the m power coefficient is calculated to be 53.333. Again (to maximize accuracy), the decimal location is shifted by 2 to the right to give a final m value of 5333 with an R coefficient of –2. Care must be taken to adjust the exponent coefficient, R, such that the value of m remains within the range of –32768 to 32767. However, rounding errors resulting from the limitations on the value of m can be mitigated by carefully selecting a slightly higher current LSB size. For example, if a Current_LSB of 1mA/bit is selected instead of 0.75mA/bit, the calculated value for m is 1 / 0.001 or 1000; because this value is a whole number there is no rounding errors and the value for R is 0. Positive values for R signify the number of times the decimal point is shifted to the left, whereas negative values for R signify the number of decimal point shifts to the right.