ATmega16U4 datasheet(300/408 Pages) ATMEL

300 / 408 page

300

7766D–AVR–01/09

ATmega16U4/ATmega32U4

The settling time and gain stage bandwidth is independent of the ADHSM bit setting.

24.5.1

ADC Input Channels

When changing channel selections, the user should observe the following guidelines to ensure

that the correct channel is selected:

• In Single Conversion mode, always select the channel before starting the conversion. The

channel selection may be changed one ADC clock cycle after writing one to ADSC. However,

the simplest method is to wait for the conversion to complete before changing the channel

selection.

• In Free Running mode, always select the channel before starting the first conversion. The

channel selection may be changed one ADC clock cycle after writing one to ADSC. However,

the simplest method is to wait for the first conversion to complete, and then change the

channel selection. Since the next conversion has already started automatically, the next

result will reflect the previous channel selection. Subsequent conversions will reflect the new

channel selection.

When switching to a differential gain channel, the first conversion result may have a poor accu-

racy due to the required settling time for the automatic offset cancellation circuitry. The user

should preferably disregard the first conversion result.

24.5.2

ADC Voltage Reference

The reference voltage for the ADC (V

REF) indicates the conversion range for the ADC. Single

ended channels that exceed V

REF will result in codes close to 0x3FF. VREF can be selected as

either AV

CC, internal 2.56V reference, or external AREF pin.

CC is connected to the ADC through a passive switch. The internal 2.56V reference is gener-

ated from the internal bandgap reference (V

BG) through an internal amplifier. In either case, the

external AREF pin is directly connected to the ADC, and the reference voltage can be made

more immune to noise by connecting a capacitor between the AREF pin and ground. V

REF can

also be measured at the AREF pin with a high impedance voltmeter. Note that V

REF is a high

impudent source, and only a capacitive load should be connected in a system.

If the user has a fixed voltage source connected to the AREF pin, the user may not use the other

reference voltage options in the application, as they will be shorted to the external voltage. If no

external voltage is applied to the AREF pin, the user may switch between AV

CC and 2.56V as

reference selection. The first ADC conversion result after switching reference voltage source

may be inaccurate, and the user is advised to discard this result.

If differential channels are used, the selected reference should not be closer to AV

CC than indi-

cated in Table on page 386.

24.6

Temperature Sensor

The ATmega16U4/ATmega32U4 includes an on-chip temperature sensor, whose the value can

be read through the A/D Converter.

The temperature measurement is based on an on-chip temperature sensor that is coupled to a

single ended ADC input. MUX[5..0] bits in ADMUX register enables the temperature sensor. The

nternal 2.56V voltage reference must also be selected for the ADC voltage reference source in

he temperature sensor measurement. When the temperature sensor is enabled, the ADC con-

verter can be used in single conversion mode to measure the voltage over the temperature

sensor.

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