Rainbow Electronics Atmega169L - Manual (Page 243)

243
ATmega169V/L
2514A–AVR–08/02
Note: 1. Incorrect setting of the switches in Figure 114 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 114. 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 105
should be used. The user is recommended not to use the Differential Gain stages dur-
ing 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. For the same reason, the ADC High
Speed mode (ADHSM) bit does not make any sense during boundary-scan operation.
The AVR ADC is based on the analog circuitry shown in Figure 114 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 approximation algorithm:
apply the lower limit on the digital DAC[9:0] lines, make sure the output from the com-
parator 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).
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 105. Boundary-scan Signals for the ADC (1) (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