
451
42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
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 ADC_CLK
tASET
A D C S ettling
C on version
S ign and M S B of R esult
LS B of R esu lt
C on version
C om plete
M U X 5:0
M U X 5:0 internal
O ld C hann el
N ew C hanne l
N ew C ha nnel
O ld C han nel
A D C In put
C han nel is
chang ed
tCHDLY
N ew C on version
can be started
fro m here
In Free Running mode MUXn can also be modified. In this case the ADC input channel
is changed after the conversion end or from the subsequent tracking phase. As a
consequence the time from one conversion to the next is extended by the duration of
the ADC settling phase.
The ADC settling time tASET depends on the previous and the new channel and on the
a synchronization delay tCHDLY from 0.5 to 2 ADC Clock cycles is required between
changing the ADC input channel selection and the beginning of the settling phase. For
If the analog input signal encounters large variations it can be useful to manually reset
the ADC and the gain amplifier before starting a new conversion. To achieve this, the
settling phase can be forced without modifying MUXn by writing a logic one to the
Analog Channel Change bit ACCH in ADCSRB
(2).
Table 27-6. Settling Time after Channel Changes
Channel Transition
Settling Time tASET in ADC Clock Cycles
To Single-Ended
ADTHT+2
To Differential
(1)
4(ADSUT+1) + 2(ADTHT+2)
Note:
1. Not allowed for ATmegaRF128RFA1 - changing to/between differential channels
requires the ADC to be disabled and enabled again.
2. The function of the ACCH bit is not available for ATmegaRF128RFA1.
27.6.3 ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC.
Single ended channels that exceed VREF will result 0x3FF. VREF 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.