Datasheet

1152
32117D–AVR-01/12
AT32UC3C
37.6.1.4 Data Registers
Data to be converted is taken from two registers, one for each channel: Data Register 0 (DR0)
for channel A and Data Register 1 (DR1) for channel B.
Alternatively both samples to be converted can be written to DR0 in a single write cycle, in this
configuration the values for channel B and A are written to the upper and the lower half words of
DR0, respectively. This operation is possible only if the DAC Dual Data in Data Register A bit of
the Configuration register (CFR.DDA) is enabled.
While the field reserved for the data to be converted is 16 bits wide, only the 12 lower bits are
considered for conversion. In order to match the expected data alignment, rounded right and left
shifts are programmable within a separate register for each data channel.
37.6.2 Advanced Operation
37.6.2.1 Prescaler
A programmable prescaler generates a divided clock signal from the system clock. This signal is
then fed to the following DACIFB programmable counters, as shown on Figure 37-2:
• The channel interval counter, which sets the minimum time interval between two samples, or
in other words, the maximum sampling frequency (see CHI bitfield in the TCR register).
• The S/H refresh counter which, in refresh mode, counts a defined number of prescaled clock
ticks (corresponding to the refresh time) before repeating the conversion of the last received
data (see CHRA and CHRB bitfields in the TCR register).
• Trigger event timer counters for both channels which, in timer trigged mode, count a defined
number of prescaled clock ticks before triggering a conversion (see the TRA and TRB
registers).
The limitations described in the “Timing constraints” paragraph must be taken into consideration
when configuring these programmable counters.
In addition to these constraints, when both channels are in use with auto-refresh mode enabled,
the refresh rate should not be significantly higher than the sampling rate on the other channel as
this might cause unexpected behavior on the latter channel.