Digital-to-analog converter (DAC) RM0440
746/2126 RM0440 Rev 4
Two calibration techniques are provided:
• Factory trimming (default setting)
The DAC buffer offset is factory trimmed. The default value of OTRIMx[4:0] bits in
DAC_CCR register is the factory trimming value and it is loaded once DAC digital
interface is reset.
• User trimming
The user trimming can be done when the operating conditions differs from nominal
factory trimming conditions and in particular when V
DDA
voltage, temperature, VREF+
values change and can be done at any point during application by software.
Note: Refer to the datasheet for more details of the Nominal factory trimming conditions
In addition, when V
DD
is removed (example the device enters in STANDBY or VBAT modes)
the calibration is required.
The steps to perform a user trimming calibration are as below:
1. If the DAC channel is active, write 0 to ENx bit in DAC_CR to disable the channel.
2. Select a mode where the buffer is enabled, by writing to DAC_MCR register,
MODEx[2:0] = 000b or 001b or 100b or 101b.
3. Start the DAC channelx calibration, by setting the CENx bit in DAC_CR register to 1.
4. Apply a trimming algorithm:
a) Write a code into OTRIMx[4:0] bits, starting by 00000b.
b) Wait for t
TRIM
delay.
c) Check if CAL_FLAGx bit in DAC_SR is set to 1.
d) If CAL_FLAGx is set to 1, the OTRIMx[4:0] trimming code is found and can be
used during device operation to compensate the output value, else increment
OTRIMx[4:0] and repeat sub-steps from (a) to (d) again.
The software algorithm may use either a successive approximation or dichotomy techniques
to compute and set the content of OTRIMx[4:0] bits in a faster way.
The commutation/toggle of CAL_FLAGx bit indicates that the offset is correctly
compensated and the corresponding trim code must be kept in the OTRIMx[4:0] bits in
DAC_CCR register.
Note: A t
TRIM
delay must be respected between the write to the OTRIMx[4:0] bits and the read of
the CAL_FLAGx bit in DAC_SR register in order to get a correct value.This parameter is
specified into datasheet electrical characteristics section.
If V
DDA
, VREF+ and temperature conditions do not change during device operation while it
enters more often in standby and VBAT mode, the software may store the OTRIMx[4:0] bits
found in the first user calibration in the flash or in back-up registers. then to load/write them
directly when the device power is back again thus avoiding to wait for a new calibration time.
When CENx bit is set, it is not allowed to set ENx bit.
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