Rainbow Electronics ATmega128RFA1 - Manual (Page 419)

419
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 27-9. ADC Timing Diagram, Changing MUXn after a Conversion
A D C C lo c k
A D IF
A D C H
A D C L
t
A S E T
A D C S e ttlin g C o n v e rs io n
S ig n a n d M S B o f R e s u lt
L S B o f R e s u lt
C o n v e rs io n
C o m p le te
M U X 5 :0
M U X 5 :0
in te rn a l
O ld C h a n n e l N e w C h a n n e l
N e w C h a n n e l O ld C h a n n e l
A D C In p u t
C h a n n e l is
c h a n g e d t
C H D L Y
N e w C o n v e rs io n
c a n b e s ta rte d
fro m h e re
Figure 27-10. ADC Timing Diagram, Changing MUXn during a Conversion
A D C C lo ck
A D IF
A D C H
A D C L
1 1 T A D C _ C L K t
A S E T
A D C S e ttlin g C o n ve rs io n
S ig n a n d M S B o f R e s u lt
L S B o f R e s u lt
C o n ve rs io n
C o m p le te
M U X 5 :0
M U X 5 :0
in te rn a l
O ld C h a n n e l N e w C h a n n e l
N e w C h a n n e l O ld C h a n n e l
A D C In p u t
C h a n n e l is
ch a n g e d
t
C H D L Y
N e w C o n v e rsio n
c a n b e sta rte d
fro m h e re
27.6.3 ADC Voltage Reference
The reference voltage for the ADC (V
REF
) indicates the conversion range for the ADC.
Single ended channels that exceed V
REF
will result 0x3FF. V
REF
can be selected by the
REFSn bits in the ADMUX register as either AVDD (1.8V), internal 1.5V or 1.6V
reference or an external voltage at the AREF pin.
AVDD is connected to the ADC through a passive switch. The internal 1.5V and 1.6V
references are generated from a bandgap reference (VBG) through an amplifier. In
either case, the external AREF pin is directly connected to the ADC and the reference
voltage can be measured at the AREF pin with a high impedance voltmeter. When
using the internal 1.5V or 1.6V references no external de-coupling capacitor must be
connected to AREF. High capacitive loading will de-stabilize the internal voltage
amplifier. The 1.6V reference voltage is calibrated to an absolute accuracy of 1 LSB
during the manufacturing process.
If the user has a fixed voltage source connected to the AREF pin, the user may not use
the other reference voltage options in the application, as they will be shorted to the
external voltage. An external reference 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 (current steps) in the course of the successive
approximation process. If no external voltage is applied to the AREF pin, the user may
switch between AVDD, 1.5V and 1.6V as reference selection.
Changes of the reference selection bits REFSn will only take effect until the first
conversion start is requested by setting ADSC in ADCSRA. After this the ADC has to be