MAC7200 datasheet(850/976 Pages) FREESCALE

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Functional Description

MAC7200 Microcontroller Family Reference Manual, Rev. 2

772

Freescale Semiconductor

3. Predischarge sample capacitor: 2 ATD

4. Sample channel (longest sampling time): 128 ATD

5. Convert channel #0 .. #31: 14 ATD

6. Adjust result & conversion shutdown: 6 SYS or 0.5 ATD

7. Conversion processing: 5 SYS

If the prescaler is smaller than 13 then phase #6 will last 6 SYS. Otherwise it will last 0.5 ATD. For this

example we assume a prescaler of five. Therefore phase #6 will last 6 SYS. The total amount of clock

cycles for this conversion will be:

2SYS + 0.5ATD + 2ATD + 128ATD + 14 ATD+ 6SYS + 5SYS

Eqn. 33-4

This sums up to:

2 SYS + 3 SYS + 10 SYS + 640 SYS + 70 SYS + 6 SYS + 5 SYS = 736 SYS

Eqn. 33-5

The whole conversion described above will need 736 system clock cycles to complete.

33.11.7 Conversions Using Predischarge

The predischarge feature allows the detection of a broken connection between a sensor and the according

ATD channel. A broken connection means that the input of an ATD channel is floating.

To detect a broken connection it is not enough to check that an ATD channel is always staying at VRH or

VRL as a floating connection can have any voltage level. Additionally when switching to the broken

channel the remaining charge from a previous channel can recharge the broken channel (phantom voltage).

By discharging the sample capacitor and the parasitic capacity of a broken ATD channel it is ensured that

the conversion result of the broken channel will be close to 0x000. Please refer to the Electrical

Specification for the maximum conversion value of a disconnected pin (CVDP).

The predischarge circuit has been designed in a way to be able to detect a broken pin between the sensor

circuit and the pad. It has not been designed to detect a broken connection in the sensor RC-filter circuit.

Depending on the dimensions of the RC-filter components it might still be possible to detect a broken

connection in the RC-filter.

The predischarge circuit consists of the following components:

•

pull-down resistor Rsc in parallel to the sample cap Csc which will discharge all parasitic capacity

between switch Smux and the RSD conversion circuit. The pull-down resistor is controlled via

switch Ssc

•

pull-down device built in the pads. The device is controlled via the ATD when the ATD module is

enabled. The pull-down device will discharge the parasitic capacity between the device pin and the

ATD channel mux. It is unavoidable that the external RC-filter circuit is discharged at the same

time.

The following figure provides further details about the predischarge circuit

MAC7200 Datasheet(PDF) 850 Page - Freescale Semiconductor, Inc

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