252
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Note:
1. Incorrect setting of the switches in
Figure 26-10 will make signal contention and may damage the part. There are
several input choices to the S&H circuitry on the negative input of the output comparator in
Figure 26-10. Make
sure only one path is selected from either one ADC pin, Bandgap reference source, or Ground.
If the ADC is not to be used during scan, the recommended input values from
Table 26-5 should be used. The user
is recommended not to use the Differential Gain stages during scan. Switch-cap based gain stages require fast
operation and accurate timing which is difficult to obtain when used in a scan chain. Details concerning operations
of the differential gain stage is therefore not provided.
The AVR ADC is based on the analog circuitry shown in
Figure 26-10 with a successive approximation algorithm
implemented in the digital logic. When used in Boundary-scan, the problem is usually to ensure that an applied
analog voltage is measured within some limits. This can easily be done without running a successive approxima-
tion algorithm: apply the lower limit on the digital DAC[9:0] lines, make sure the output from the comparator is low,
then apply the upper limit on the digital DAC[9:0] lines, and verify the output from the comparator to be high.
The ADC needs not be used for pure connectivity testing, since all analog inputs are shared with a digital port pin
as well.
When using the ADC, remember the following:
The Port Pin for the ADC channel in use must be configured to be an input with pull-up disabled to avoid signal
contention.
In Normal mode, a dummy conversion (consisting of 10 comparisons) is performed when enabling the ADC.
The user is advised to wait at least 200 ns after enabling the ADC before controlling/observing any ADC signal,
or perform a dummy conversion before using the first result.
The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal low (Sample mode).
MUXEN_0
Input
Input Mux bit 0
1
NEGSEL_2
Input
Input Mux for negative input for differential
signal, bit 2
00
NEGSEL_1
Input
Input Mux for negative input for differential
signal, bit 1
00
NEGSEL_0
Input
Input Mux for negative input for differential
signal, bit 0
00
PASSEN
Input
Enable pass-gate of gain stages.
1
PRECH
Input
Precharge output latch of comparator
(Active low)
11
SCTEST
Input
Switch-cap TEST enable. Output from x10
gain stage send out to Port Pin having
ADC_4
00
ST
Input
Output of gain stages will settle faster if this
signal is high first two ACLK periods after
AMPEN goes high.
00
VCCREN
Input
Selects Vcc as the ACC reference voltage.
0
Table 26-5.
Boundary-scan Signals for the ADC
(1) (Continued)
Signal
Name
Direction
as Seen
from the
ADC
Description
Recommended
Input when not
in Use
Output Values when
Recommended Inputs
are Used, and CPU is
not Using the ADC