mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 664

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Functional description
When calibration is complete, the user may reconfigure and use the ADC as desired. A
second calibration may also be performed if desired by clearing and again setting the
CAL bit.
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 (offset,
plus-side gain, and plus-side calibration values) may be 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.
29.4.7 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
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, 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.
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.
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3. Divide the variable by two.
4. Set the MSB of the variable.
5. The previous two steps can be achieved by setting the carry bit, rotating to the right
6. Store the value in the plus-side gain calibration register (PG).
through the carry bit on the high byte and again on the low byte.
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Preliminary
Freescale Semiconductor, Inc.