Dma Request; Noise Generation; Figure 44. Dac Lfsr Register Calculation Algorithm - ST STM32F101xx Reference Manual

Digital-to-analog converter (DAC)
Note: 1 TSELx[2:0] bit cannot be changed when the ENx bit is set.
2 When software trigger is selected, it takes only one APB1 clock cycle for DAC_DHRx-to-
DAC_DORx register transfer.
12.3.7

DMA request

Each DAC channel has a DMA capability. Two DMA channels are used to service DAC
channel DMA requests.
A DAC DMA request is generated when an external trigger (but not a software trigger)
occurs while the DMAENx bit is set. The value of the DAC_DHRx register is then transferred
to the DAC_DORx register.
In dual mode, if both DMAENx bits are set, two DMA requests are generated. If only one
DMA request is needed, you should set only the corresponding DMAENx bit. In this way, the
application can manage both DAC channels in dual mode by using one DMA request and a
unique DMA channel.
The DAC DMA request is not queued so that if a second external trigger arrives before the
acknowledgement of the last request, then the new request will not be serviced and no error
is reported
12.3.8

Noise generation

In order to generate a variable-amplitude pseudonoise, a Linear Feedback Shift Register is
available. The DAC noise generation is selected by setting WAVEx[1:0] to "01". The
preloaded value in the LFSR is 0xAAA. This register is updated, three APB1 clock cycles
after each trigger event, following a specific calculation algorithm.

Figure 44. DAC LFSR register calculation algorithm

The LFSR value, that may be masked partially or totally by means of the MAMPx[3:0] bits in
the DAC_CR register, is added up to the DAC_DHRx contents without overflow and this
value is then stored into the DAC_DORx register.
If LFSR is 0x0000, a '1' is injected into it (antilock-up mechanism).
It is possible to reset LFSR wave generation by resetting the WAVEx[1:0] bits.
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