MAC7200 datasheet(851/976 Pages) FREESCALE

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Functional Description

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

773

Figure 33-21. Predischarge Circuit

Before a conversion starts the sampling phase it can start the predischarge phase (see Section 33.11.6,

“Conversion Timing” for details about the various conversion phases). The predischarge phase is only

executed when the ATDMODE.PRED bit is set. Otherwise the phase is skipped. The length of the

predischarge phase can be set by the ATDPTS.TIME bits. It can be set between 1 and 16 ATD clock cycles.

Which predischarge time needs to be selected depends on various factors e.g.:

•

resistance of the PAD pull-down device (voltage dependent)

•

parasitic capacity of the device pin (solder joint etc.)

Discharging the parasitic capacity will also discharge the external RC-filter. Therefore the length of the

predischarge and the sampling phase should be selected in a way which allows the sensor to restore the

filter capacitor voltage that was available before the predischarge phase started. Otherwise the conversion

result value (ATDRR.RRCR) will be lower.

Figure 33-22 shows one timing example for a predischarge phase.:

Figure 33-22. Predischarge Timing

In the example shown in Figure 33-22 for the predischarge phase length three ATD clock cycles were

selected (ATDPTS.TIME = 0b0010). For the sampling phase four ATD clock cycles were selected.

Sensor

Csc

Rsc

AN0

AN31

Sensor circuit

Pad pull-down

Pull enable

RSD

conversion

circuit

ATD

(chip-external)

Smux

Ssc

Device pin

device

predischarge phase

sampling phase

start

idle

conversion phase

ATD clock

Pull enable / Ssc

Conversion state

Smux

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