ST62E25BM3 datasheet(54/105 Pages) STMICROELECTRONICS

No. de pieza	ST62E25BM3

Descripción Electrónicos	8-bit MCUs with A/D converter

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Fabricante Electrónico	STMICROELECTRONICS [STMicroelectronics]

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A/D CONVERTER (Cont’d)

9.3.3 Functional Description

9.3.3.1 Analog Power Supply

The high and low level reference voltage pins are

internally connected to the VDD and VSS pins.

Conversion accuracy may therefore be impacted

by voltage drops and noise in the event of heavily

loaded or badly decoupled power supply lines.

9.3.3.2 Digital A/D Conversion Result

The conversion is monotonic, meaning that the re-

sult never decreases if the analog input does not

and never increases if the analog input does not.

If the input voltage (VAIN) is greater than or equal

to VDDA (high-level voltage reference) then the

conversion result in the DR register is FFh (full

scale) without overflow indication.

If input voltage (VAIN) is lower than or equal to

VSSA (low-level voltage reference) then the con-

version result in the DR register is 00h.

The A/D converter is linear and the digital result of

the conversion is stored in the ADR register. The

accuracy of the conversion is described in the par-

ametric section.

RAIN is the maximum recommended impedance

for an analog input signal. If the impedance is too

high, this will result in a loss of accuracy due to

leakage and sampling not being completed in the

allocated time. Refer to the electrical characteris-

tics chapter for more details.

With an oscillator clock frequency less than

1.2MHz, conversion accuracy is decreased.

9.3.3.3 Analog Input Selection

Selection of the input pin is done by configuring

the related I/O line as an analog input via the Data

Direction, Option and Data registers (refer to I/O

ports description for additional information).

Caution: Only one I/O line must be configured as

an analog input at any time. The user must avoid

any situation in which more than one I/O pin is se-

lected as an analog input simultaneously, because

they will be shorted internally.

9.3.3.4 Software Procedure

Refer to the Control register (ADCR) and Data reg-

ister (ADR) in Section 9.3.7 for the bit definitions.

Analog Input Configuration

The analog input must be configured through the

Port Control registers (DDRx, ORx and DRx). Re-

fer to the I/O port chapter.

ADC Configuration

In the ADCR register:

– Reset the PDS bit to power on the ADC. This bit

must be set at least one instruction before the

beginning of the conversion to allow stabilisation

of the A/D converter.

– Set the EAI bit to enable the ADC interrupt if

needed.

ADC Conversion

In the ADCR register:

– Set the STA bit to start a conversion. This auto-

matically clears (resets to “0”) the End Of Con-

version Bit (EOC).

When a conversion is complete

– The EOC bit is set by hardware to flag that con-

version is complete and that the data in the ADC

data conversion register is valid.

– An interrupt is generated if the EAI bit was set

Setting the STA bit will start a new count and will

clear the EOC bit (thus clearing the interrupt con-

dition)

Note:

Setting the STA bit must be done by a different in-

struction from the instruction that powers-on the

ADC (setting the PDS bit) in order to make sure

the voltage to be converted is present on the pin.

Each conversion has to be separately initiated by

writing to the STA bit.

The STA bit is continuously scanned so that, if the

user sets it to “1” while a previous conversion is in

progress, a new conversion is started before com-

pleting the previous one. The start bit (STA) is a

write only bit, any attempt to read it will show a log-

ical “0”.

1

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