AN1636 datasheet(39/42 Pages) STMICROELECTRONICS

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UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

Quartz crystals must be surrounded by ground tracks/plane. The other side of the two layer

PCB below the crystal should preferably be covered by the ground plane. Most crystals have

a metallic body that should be grounded. The crystal should be placed close to the microcon-

troller. Surface mounted crystals are available and can be used.

6. Software considerations

Do not toggle digital outputs on the same I/O port as the A/D input being converted. This will

introduce switching noise into the analog inputs.

Toggling high sink I/Os may introduce high frequency noise in the power supply, this may af-

fect the conversion results.

7 HOW POWER SAVING MODES AFFECT THE ADC

Wait mode

ADC can be used when the microcontroller is in Wait mode. You can take advantage of the re-

duced internal noise in the microcontroller to improve ADC conversion results.

Halt mode

Entering Halt mode will disable the ADC, irrespective of whether the ADON bit is set or not.

This is because executing the HALT instruction shuts down the ADC.

Exiting from Halt mode requires some stabilization time before ADC conversion should be

started. This conversion time is very small. Please refer to the datasheets.

Table 3. Summary Table

ST7 Modes

Oscillator/CPU/Peripheral Status

Effect on ADC

Oscil-

lator

CPU

MCCSR-

OIE Bit

MCCSR-

SMS Bit

ADC

Run

Reset

Use the ADC as mentioned in da-

tasheet.

Slow

Set

Take care that fADC does not fall

below fADC min.

Wait

Off

Reset

If ADC has exit-from Wait capa-

bility, this mode is recommended.

Slow-Wait

Off

Set

If ADC has exit-from Wait capa-

bility, this mode is recommended.

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