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GR716-DS-UM, May 2019, Version 1.29
115
www.cobham.com/gaisler
GR716
15.2
Operation
15.2.1 System overview
There are four independent 12bit/3MSps DAC blocks. They have sourcing-current single-ended out-
puts, typically to be loaded by virtual grounds generated by op-amps on PCB, or by passive imped-
ances connected to PCB ground providing the output voltages directly across these impedances.
These four blocks are supplied by VDDA_DAC and VSSA_DAC. In the same way as for the ADC, it
is enough to provide really good decoupling at high frequencies (>~1MHz).
Note that the DAC fullscale current is proportional to the current through the external RREF, and
5.11kohm gives a fullscale current of 4.0mA nominally.
15.2.2 Detailed description
Each DAC will convert a 12 bit register to an analog output. The register is accessible via the APB
register interface the digital DAC interface provides. The conversion from the 12 bit register can take
affect immediately or when a selected trigger event occurs. The DSEQ.SQ bit determines the DAC
mode. When DSEQ.SQ is set to ’0’ the conversion will take affect immediately and when DSEQ.SQ
is set to ’1’ the conversion will take affect on the next trigger event selected in the DSYNC register.
Triggers can be set to synchronize outputs from the DAC. When triggers are used the DAC output
level or value will not be updated or changed until an event has occurred on the selected trigger. The
trigger event can be programmed to also generate an interrupt when a new value from the processor
can be accepted. When the trigger event has occurred the status register is updated regardless of the
interrupt generation.
In trigger mode the output value in register DOUT.DI can be updated at anytime without affecting the
DAC output. The value in the 12 bit register is directly forwarded to the analog DAC when an trigger
event occur.
The speed of the conversion can be in the range from 1Ksps to 3Msps. The conversion rate is set by
the internal DAC scaler register field DCFG.DS. The register field is calculated by dividing the sys-
tem clock frequency with sample rate.
Slew rate control can be enabled for system with sampling conversion rate up to 50Ksps. Enabling
slew rate control will limit the DAC output current change and improve noise on the output. Slew rate
control is enabled with register field DCFG.DD.
15.2.3 DAC output example
To enable DAC and direction conversation of the register use the following steps:
DCFG.DE = 8
// Enable DAC
DCFG.DS = 0x1F4 // Set DAC scaler
DOUT.DI = 1 // DAC digital input value
15.2.4 Access control
The integrated DAC is controlled via APB registers
TABLE 105. Example of direction conversion of value 0xFF using DAC0
Address
Data
Description
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0x80408004
0x000000FF
DAC0 - Output Value
0x80408000
0x01FCC001
DAC0 - Configuration (Scaler, Mode, Enable)
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