Rainbow-electronics ATmega128RFA1 Bedienungsanleitung Seite 411

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411
8266A-MCU Wireless-12/09
ATmega128RFA1
The Power Reduction ADC bit, PRADC (see "PRR0 Power Reduction Register0" on
page 167) must be disabled by writing a logical zero to enable the ADC.
Figure 27-1. Analog to Digital Converter Block Schematic
8-B IT D ATA BU S
ADC CTRL & STATUS
REGISTE R B (ADC SR B)
ADC CTRL & STATU S
REG ISTER C (ADC SR C)
ADC M ULTIPLEXE R
SELECT (ADMUX)
ADC CTRL & STATUS
REGISTER A (ADCSR A)
TRIGGE R
SELECT
ADFR
ADSC
ADTS[2:0]
IN TERRUP T
FLAGS
ADIF
ADIE
AD CO NVER SION
CO MPLETE IR Q
ADC DATA REG ISTER
(ADC H/ADCL)
15 0
PRESC ALE R
CO NVER SION LOGIC
START
ADPS[2:0]
ADIF
ADEN
ADC[9:0]
ADSUT[4:0]
ADTHT[1:0]
ADLAR
MUX DECODER
MUX[4:0]
MUX[5]
REFS[1:0]
CHANNEL SELECTION
DIFF / GAIN SELECT
10-bit DAC
SAMPLE & H OLD
CO MPARATOR
AVDD
AREF
INT E R NAL
REFER ENCE
(1.5V/1.6 V)
ADC[7:0]
AVSS
BANDG AP
REFERE NCE
1.2V
TEMPE RATURE
SENS O R
ADC[2:0]
GAIN
AMPLIFIE R
ADC
MULTIPLEXER
OUTPUT
DRT VO LTAGE
SRAM 2
CLAMP
ACCH
27.2 Operation
The ADC converts an analog input voltage to a 10-bit digital value through successive
approximation. The minimum value represents 0V (conversion result 0x000) and the
maximum value in single ended mode represents the reference voltage minus 1 LSB
(conversion result 0x3FF). The reference voltage can be measured at the AREF pin.
The internal, generated reference voltage can have the values 1.5V, 1.6V or AVDD
where the 1.6V has the highest absolute accuracy. The reference voltage is selected by
writing to the REFSn bits in the ADMUX Register. An external reference voltage can
also be selected. Such an external voltage must be supplied with a very low impedance
R
AREF,EXT
(see "ADC Electrical Characteristics" on page 505). The load current I
L,AREF
(see "ADC Electrical Characteristics" on page 505) seen by the external source is code
dependent and changes in the course of the successive approximation process (load
current steps). The internal voltage reference (except AVDD) must not be decoupled by
an external capacitor. Adding unnecessary external capacitance at the AREF pin will
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