MAC7200 datasheet(860/976 Pages) FREESCALE

860 / 976 page

Initialization/Application Information

MAC7200 Microcontroller Family Reference Manual, Rev. 2

782

Freescale Semiconductor

After OCC and GCC have been calculated they must be written to the ATDCAL register. Now each raw

conversion result can be automatically adjusted by setting the ATDCW[CWAR] bit.

It is advisable to re-convert the reference channels using the new calibration factors like described in

33.12.2.3.1, “Sample Special Channel 75% x (VRH - VRL) and in 33.12.2.3.2, “Sample Special Channel

25% x (VRH - VRL). The results should match the ideal 25% and 75% results plus a margin that depends

upon the noise on the system.

33.12.2.3.4

Adjustment Example

For a better understanding how the single steps mentioned in Section 33.12.2, “ATD Calibration / Result

Adjustment work an example is provided in this section:

It is assumed that the (averaged) raw results from the 75% x (VRH - VRL) and the 25% x (VRH - VRL)

conversion are 2951 and 951. Therefore the gain is calculated as:

Eqn. 33-16

The gain calibration constant (GCC) is calculated as:

Eqn. 33-17

When calculating GCC it is highly recommended to use truncation instead of rounding.

The offset is calculated as:

Eqn. 33-18

As the offset is positive the offset calibration constant (OCC) is calculated as:

Eqn. 33-19

When calculating OCC it is highly recommended to use truncation instead of rounding.

Now OCC and GCC can be written to the ATDCAL register. To verify that the two constants have been

calculated correctly 75% x (VRH - VRL) and 25% x (VRH - VRL) can be converted again. This time the

CWAR bit in the command word must be set. The results should be 3072 and 1024.

33.12.3 Conversion Examples

This section provides some conversion examples which should clarify the ATD working behavior. The

following examples are given:

•

Example 1: A Simple Conversion

•

Example 2: A Simple Conversion Sequence (Convert then Pause)

•

Example 3: Interrupted Conversion Sequence

•

Example 4: Edge-triggered Conversion

•

Example 5: Level-triggered Conversion

•

Example 6: Queue Running Idle

Gain

2048

RAW

–

---------------------------------------------

2048

2951

951

–

-----------------------------

2048

2000

-------------

1.024

GCC

Gain

16384

16777.2

16777

≈

Offset

3072

Gain

RAW

–

0.5

–

3072

2048

2000

-------------

2951

–

0.5

–

49.676

OCC

Offset

49.676

198.70

198

≈

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