Atmel ATmega128, Manual

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2467S–AVR–07/09
ATmega128
Note: Incorrect setting of the switches in Figure 131 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 com-
parator in
Figure 131 . 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 104 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 there-
fore not provided.
The AVR ADC is based on the analog circuitry shown in Figure 131 with a successive approxi-
mation 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 digi-
tal 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
.
SCTEST Input Switch-cap TEST
enable. Output from
x10 gain stage send
out to Port Pin having
ADC_4 0 0
ST Input Output of gain stages
will settle faster if this
signal is high first two
ACLK periods after
AMPEN goes high. 0 0
VCCREN Input Selects Vcc as the
ACC reference
voltage. 0 0
Table 104. Boundary-scan Signals for the ADC (Continued)
Signal
Name
Direction
as Seen
from the
ADC
Description
Recommen-
ded 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 5
V
⁄ ⋅ ⋅
291 0x123 = =
The upper limit is: 1024 1.5
V
1.05 5
V
⁄ ⋅ ⋅
323 0x143
= =