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1745D–ATARM–04-Nov-05
AT91M55800A
DA = DAVREF x (DAC_DOR / 1024)
When DAC_DOR (Data Output Register) is loaded, the analog output voltage is available after
a settling time of approximately 5 µsec. The exact value depends on the power supply voltage
and the analog output load, and is indicated in the Electrical Characteristics Sheet of the
device as parameter t
DAST
.
The output register cannot be written directly and any data transfer to the DAC must be per-
formed by writing in DAC_DHR (Data Holding Register). The transfer from DAC_DHR to
DAC_DOR is performed automatically or when an hardware trigger occurs, depending on the
bit TRGEN in DAC_MR (Mode Register).
The DAC integrates an output buffer enabling the reduction of the output impedance, and the
possibility of driving external loads directly, without having to add an external operational
amplifier. The maximum load supported by the output buffer is indicated in the Electrical Char-
acteristics of the device.
22.1.1 8- or 10-bit Conversion Mode
Bit RES in the Mode Register (DAC_MR) selects between 8-bit or 10-bit modes of operation.
In 8-bit mode, the data written in DAC_DHR is automatically shifted left two bits and the two
lowest bits are written 0. The bit RES also affects the type of transfers performed by the PDC
channel:
• in 8-bit mode, only a byte transfer is performed .
• in 10-bit mode, a half-word transfer (16 bits) is performed.
22.1.2 Trigger Selection
A conversion is triggered when data is written in DAC_DHR and TRGEN in DAC_MR is 0.
If TRGEN is 1, a hardware trigger is selected by the field TTRGSEL between the Timer
Counter Channel outputs TIOA. In this case, the corresponding Timer Counter channel must
be programmed in Waveform Mode, and each time the DAC detects a rising edge on the TC
output, it transfers the last data written in DAC_DHR into DAC_DOR.
The bit DATRDY traces the fact that a valid data has been written in DAC_DHR and not yet
been transferred in DAC_DOR. An interrupt can be generated from this status bit to tell the
software to load the following value.