Datasheet

304
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
When switching to a differential gain channel, the first conversion result may have a poor accuracy 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. V
REF
can be selected as either AV
CC
, internal 2.56V
reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 2.56V reference is generated 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 indicated in Table 29-
7 on page 390.
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 internal 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 converter can be used in single conversion mode to
measure the voltage over the temperature sensor.
The temperature sensor and its internal driver are enabled when ADMUX value selects the temperature sensor
as ADC input. The propagation delay of this driver is approximately 2µS. Therefore two successive conversions
are required. The correct temperature measurement will be the second one.
One can also reduce this timing to one conversion by setting the ADMUX during the previous conversion.
Indeed the ADMUX can be programmed to select the temperature sensor just after the beginning of the
previous conversion start event and then the driver will be enabled 2µS before sampling and hold phase of
temperature sensor measurement.
24.6.1 Sensor Calibration
The sensor initial tolerance is large (±10°C), but its characteristic is linear. Thus, if the application requires
accuracy, the firmware must include a calibration stage to use the sensor for direct temperature measurement.
Another application of this sensor may concern the Internal Calibrated RC Oscillator, whose the frequency can
be adjusted by the user through the OSCCAL register (see “Oscillator Calibration Register – OSCCAL” on
page 32). During the production, a calibration is done at two temperatures (+25°C and +85°C, with a tolerance
of ±10°C
(1)
). At each temperature, the temperature sensor value T
i
is measured and stored in EEPROM
memory
(2)
, and the OSCCAL calibration value O
i
(i.e. the value that should be set in OSCCAL register at this
temperature to have an accurate 8MHz output) is stored in another memory zone.