Rainbow Electronics ATmega64M1 - Manual (Page 267)

267
8209A–AVR–08/09
ATmega16M1/32M1/64M1
25.4 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.
25.4.1 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. In either case, the
external AREF pin is directly connected to the DAC, 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 impedant voltmeter. Note that V
REF
is a high
impedant 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 DAC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
25.5 Register Description
The DAC is controlled via three dedicated registers, the DACON register which is used for DAC
configuration, and the DACH and DACL which are used to set the value to be converted.
25.5.1 DACON – Digital to Analog Conversion Control Register
• Bit 7 – DAATE: DAC Auto Trigger Enable bit
Set this bit to update the DAC input value on the positive edge of the trigger signal selected with
the DACTS2-0 bit in DACON register.
Clear it to automatically update the DAC input when a value is written on DACH register.
Bit 7 6 5 4 3 2 1 0
DAATE DATS2 DATS1 DATS0 - DALA DAOE DAEN DACON
Read/Write R/W R/W R/W R/W - R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0