ATmega32A datasheet(245/352 Pages) ATMEL

245 / 352 page

245

8155B–AVR–07/09

ATmega32A

If the ADC is not to be used during scan, the recommended input values from Table 24-5 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 there-

fore not provided.

The AVR ADC is based on the analog circuitry shown in Figure 24-10 with a successive approx-

imation 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 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).

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.

The recommended values from Table 24-5 are used unless other values are given in the algo-

rithm in Table 24-6. Only the DAC and Port Pin values of the Scan-chain are shown. The column

“Actions” describes what JTAG instruction to be used before filling the Boundary-scan Register

with the succeeding columns. The verification should be done on the data scanned out when

scanning in the data on the same row in the table.

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

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