250
8285B–AVR–03/11
ATmega165A/165PA/325A/325PA/3250A/3250PA/645A/645P/645
Note:
may damage the part. There are several input choices to the S&H circuitry on the negative
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 24-3 onpage 248 should be used. The user is recommended not to use the Differential Amplifier during
scan. Switch-Cap based differential amplifier requires fast operation and accurate timing which
is difficult to obtain when used in a scan chain. Details concerning operations of the differential
amplifier is therefore not provided.
sive 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 approximation 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 need 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 200ns 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).
As an example, consider the task of verifying a 1.5V ± 5% input signal at ADC channel 3 when
the power supply is 5.0V and AREF is externally connected to V
CC.
PRECH
Input
Precharge output latch of
comparator. (Active low)
11
SCTEST
Input
Switch-cap TEST enable. Output
from differential amplifier is sent out
to Port Pin having ADC_4
00
ST
Input
Output of differential amplifier 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.
00
Table 24-3.
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
The lower limit is:
1024 1.5
V 0,95 5V
291
0x123
==
The upper limit is:
1024 1.5
V 1.05 5V
323
0x143
==