Datasheet

228
7734Q–AVR–02/12
AT90PWM81/161
18.2 Operation
The Digital to Analog Converter generates an analog signal proportional to the value of the DAC
registers value.
In order to have an accurate sampling frequency control, there is the possibility to update the
DAC input values through different trigger events.
18.3 Starting a Conversion
The DAC is configured thanks to the DACON register. As soon as the DAEN bit in DACON reg-
ister is set, the DAC converts the value present on the DACH and DACL registers in accordance
with the register DACON setting.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is
enabled by setting the DAC Auto Trigger Enable bit, DAATE in DACON. The trigger source is
selected by setting the DAC Trigger Select bits, DATS in DACON (see description of the DATS
bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal,
the DAC converts the value present on the DACH and DACL registers in accordance with the
register DACON setting. This provides a method of starting conversions at fixed intervals. If the
trigger signal is still set when the conversion completes, a new conversion will not be started. If
another positive edge occurs on the trigger signal during conversion, the edge will be ignored.
Note that an interrupt flag will be set even if the specific interrupt is disabled or the Global Inter-
rupt Enable bit in SREG is cleared. A conversion can thus be triggered without causing an
interrupt. However, the interrupt flag must be cleared in order to trigger a new conversion at the
next interrupt event.
18.3.1 DAC Voltage Reference
The reference voltage for the ADC (V
REF
) indicates the conversion range for the DAC. V
REF
can
be selected as either AV
CC
, internal 2.56V reference, or external AREF pin.
AV
CC
is connected to the DAC through a passive switch. The internal 2.56V reference is gener-
ated from the internal bandgap reference (V
BG
) through an internal amplifier. When the external
AREF pin is connected to the DAC, 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 impedance source, and only
a capacitive load should be connected in a system.
The user may switch between AV
CC,
AV
CC
and 2.56V as reference selection. The first DAC con-
version result after switching reference voltage source may be inaccurate, and the user is
advised to discard this result.
18.4 DAC Register Description
The DAC is controlled via three dedicated registers:
• The DACON register which is used for DAC configuration
• DACH and DACL which are used to set the value to be converted
18.4.1 DACON - Digital to Analog Conversion Control Register
Bit 7 6543210
DAATE DATS2 DATS1 DATS0 - DALA - DAEN DACON
Read/Write R/W R/W R/W R/W - R/W - R/W
Initial Value 0 0 0 0 0 0 0 0