ATA8741-PXQW【PDF 156/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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156

9140CINDCO02/10

ATA8741

the MUX0 bit in ADMUX. This amplified value then becomes the analog input to the ADC. If sin-

gle ended channels are used, the gain amplifier is bypassed altogether.

The offset of the differential channels can be measure by selecting the same input for both neg-

ative and positive input. Offset calibration can be done for ADC0, ADC3 and ADC7. When ADC0

or ADC3 or ADC7 is selected as both the positive and negative input to the differential gain

amplifier , the remaining offset in the gain stage and conversion circuitry can be measured

directly as the result of the conversion. This figure can be subtracted from subsequent conver-

sions with the same gain setting to reduce offset error to below 1 LSB.

The on-chip temperature sensor is selected by writing the code 100010 to the MUX5..0 bits in

ADMUX register.

The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and

input channel selections will not go into effect until ADEN is set. The ADC does not consume

power when ADEN is cleared, so it is recommended to switch off the ADC before entering power

saving sleep modes.

The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and

ADCL. By default, the result is presented right adjusted, but can optionally be presented left

adjusted by setting the ADLAR bit in ADCSRB.

If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read

ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the data

registers belongs to the same conversion. Once ADCL is read, ADC access to data registers is

blocked. This means that if ADCL has been read, and a conversion completes before ADCH is

read, neither register is updated and the result from the conversion is lost. When ADCH is read,

ADC access to the ADCH and ADCL Registers is re-enabled.

The ADC has its own interrupt which can be triggered when a conversion completes. When ADC

access to the data registers is prohibited between reading of ADCH and ADCL, the interrupt will

trigger even if the result is lost.

25.4 Starting a Conversion

A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC.

This bit stays high as long as the conversion is in progress and will be cleared by hardware

when the conversion is completed. If a different data channel is selected while a conversion is in

progress, the ADC will finish the current conversion before performing the channel change.

Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is

enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is

selected by setting the ADC Trigger Select bits, ADTS in ADCSRB (see description of the ADTS

bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal,

the ADC prescaler is reset and a conversion is started. This provides a method of starting con-

versions at fixed intervals. If the trigger signal still is set when the conversion completes, a new

conversion will not be started. If another positive edge occurs on the trigger signal during con-

version, the edge will be ignored. Note that an Interrupt Flag will be set even if the specific

interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus

be triggered without causing an interrupt. However, the Interrupt Flag must be cleared in order to

trigger a new conversion at the next interrupt event.

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