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Chapter 23 Analog-to-Digital Converter (ADC)
Overall, the calibration routine may take as many as 14k ADCK cycles and 100 bus
cycles, depending on the results and the clock source chosen. For an 8 MHz clock source,
this length amounts to about 1.7 ms. To reduce this latency, the calibration values, which
are offset, plus-side and minus-side gain, and plus-side and minus-side calibration values,
may be stored in flash memory after an initial calibration and recovered prior to the first
ADC conversion. This method can reduce the calibration latency to 20 register store
operations on all subsequent power, reset, or Low-Power Stop mode recoveries.
Further information on the calibration procedure can be found in the Calibration section
of
AN3949: ADC16 Calibration Procedure and Programmable Delay Block
Synchronization.
23.5.7 User-defined offset function
OFS contains the user-selected or calibration-generated offset error correction value.
This register is a 2's complement, left-justified. The value in OFS is subtracted from the
conversion and the result is transferred into the result registers, Rn. If the result is greater
than the maximum or less than the minimum result value, it is forced to the appropriate
limit for the current mode of operation.
The formatting of the OFS is different from the data result register, Rn, to preserve the
resolution of the calibration value regardless of the conversion mode selected. Lower
order bits are ignored in lower resolution modes. For example, in 8-bit single-ended
mode, OFS[14:7] are subtracted from D[7:0]; OFS[15] indicates the sign (negative
numbers are effectively added to the result) and OFS[6:0] are ignored. The same bits are
used in 9-bit differential mode because OFS[15] indicates the sign bit, which maps to
D[8]. For 16-bit differential mode, OFS[15:0] are directly subtracted from the conversion
result data D[15:0]. In 16-bit single-ended mode, there is no field in the OFS
corresponding to the least significant result D[0], so odd values, such as -1 or +1, cannot
be subtracted from the result.
OFS is automatically set according to calibration requirements once the self-calibration
sequence is done, that is, SC3[CAL] is cleared. The user may write to OFS to override
the calibration result if desired. If the OFS is written by the user to a value that is
different from the calibration value, the ADC error specifications may not be met. Storing
the value generated by the calibration function in memory before overwriting with a user-
specified value is recommended.
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
Freescale Semiconductor, Inc.
377
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