Information

stored in flash memory after an initial calibration and recovered prior to the first ADC

conversion. This method should reduce the calibration latency to 20 register store

operations on all subsequent power, reset, or Low Power Stop mode recoveries.

31.4.7 User defined offset function

The ADC offset correction register (OFS) contains the user selected or calibration

generated offset error correction value. This register is a 2’s complement, left justified.

The value in the offset correction register (OFS) is subtracted from the conversion and

the result is transferred into the result registers (Rn). If the result is above the maximum

or below the minimum result value, it is forced to the appropriate limit for the current

mode of operation.

The formatting of the ADC offset correction register is different from the data result

conversion mode selected. Lower order bits are ignored in lower resolution modes. For

example, in 8-bit single-ended mode, the bits OFS[14:7] are subtracted from D[7:0]; bit

OFS[15] indicates the sign (negative numbers are effectively added to the result) and bits

OFS[6:0] are ignored. The same bits are used in 9-bit differential mode since bit OFS[15]

indicates the sign bit, which maps to bit D[8]. For 16-bit differential mode, all bits

OFS[15:0] are directly subtracted from the conversion result data D[15:0]. In 16-bit

single-ended mode, there is no bit in the offset correction register corresponding to the

least significant result bit D[0], so odd values (-1 or +1, and so on) cannot be subtracted

from the result.

OFS is automatically set according to calibration requirements once the self calibration

sequence is done (CAL is cleared). The user may write to OFS to override the calibration

result if desired. If the offset correction register is written by the user to a value that is

different from the calibration value, the ADC error specifications may not be met. It is

recommended that the value generated by the calibration function be stored in memory

before overwriting with a user specified value.

Note

There is an effective limit to the values of offset that can be set

by the user. If the magnitude of the offset is too great, the

results of the conversions will cap off at the limits.

The offset calibration function may be employed by the user to remove application

offsets or DC bias values. The offset correction register, OFS may be written with a

number in 2's complement format and this offset will be subtracted from the result (or

hardware averaged value). To add an offset, store the negative offset in 2's complement

Functional description

K20 Sub-Family Reference Manual, Rev. 2, Feb 2012

620 Freescale Semiconductor, Inc.