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9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

18.11.2

ADCSRA – ADC Control and Status Register A

• Bit 7 – ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the

ADC off while a conversion is in progress, will terminate this conversion.

• Bit 6 – ADSC: ADC Start Conversion

In Single Conversion mode, write this bit to one to start each conversion. In Free Running

mode, write this bit to one to start the first conversion. The first conversion after ADSC has

been written after the ADC has been enabled, or if ADSC is written at the same time as the

ADC is enabled, will take 25 ADC clock cycles instead of the normal 13. This first conversion

performs initialization of the ADC.

ADSC will read as one as long as a conversion is in progress. When the conversion is com-

plete, it returns to zero. Writing zero to this bit has no effect.

• Bit 5 – ADATE: ADC Auto Trigger Enable

When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a

conversion on a positive edge of the selected trigger signal. The trigger source is selected by

setting the ADC Trigger Select bits, ADTS in ADCSRB.

• Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the data registers are updated. The

ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set.

ADIF is cleared by hardware when executing the corresponding interrupt handling vector.

Alternatively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a

Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also applies if the

SBI and CBI instructions are used.

• Bit 3 – ADIE: ADC Interrupt Enable

When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete

Interrupt is activated.

• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the system clock frequency and the input

clock to the ADC.

Bit

ADEN

ADSC

ADATE

ADIF

ADIE

ADPS2

ADPS1

ADPS0

ADCSRA

Read/Write

R/W

Initial Value

Table 18-6.

ADC Prescaler Selections

ADPS2

ADPS1

ADPS0

Division Factor

000

001

010

011

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