
DocID024597 Rev 5
721/1830
RM0351
Digital filter for sigma delta modulators (DFSDM)
756
There are 2 options for comparing the threshold registers with the data values
•
Option1: in this case, the input data are taken from final output data register
(AWFSEL=0). This option is characterized by:
–
high input data resolution (up to 24-bits)
–
slow response time - inappropriate for fast response applications like overcurrent
detection
–
for the comparison the final data are taken after bit shifting and offset data
correction
–
final data are available only after main regular or injected conversions are
performed
–
can be used in case of parallel input data source (DATMPX[1:0]
≠
0 in
DFSDM_CHyCFGR1 register)
•
Option2: in this case, the input data are taken from any serial data receivers output
(AWFSEL=1). This option is characterized by:
–
input serial data are processed by dedicated analog watchdog Sinc
x
channel
filters with configurable oversampling ratio (1..32) and filter order (1..3) (see
AWFOSR[4:0] and AWFORD[1:0] bits setting in DFSDM_CHyAWSCDR register)
–
lower resolution (up to 16-bit)
–
fast response time - appropriate for applications which require a fast response like
overcurrent/overvoltage detection)
–
data are available in continuous mode independently from main regular or injected
conversions activity
In case of input channels monitoring (AWFSEL=1), the data for comparison to threshold is
taken from channels selected by AWDCH[7:0] field (DFSDM_FLTxCR2 register). Each of
the selected channels filter result is compared to one threshold value pair (AWHT[23:0] /
AWLT[23:0]). In this case, only higher 16 bits (AWHT[23:8] / AWLT[23:8]) define the 16-bit
threshold compared with the analog watchdog filter output because data coming from the
analog watchdog filter is up to a 16-bit resolution. Bits AWHT[7:0] / AWLT[7:0] are not taken
into comparison in this case (AWFSEL=1).
Parameters of the analog watchdog filter configuration for each input channel are set in
DFSDM_CHyAWSCDR register (filter order AWFORD[1:0] and filter oversampling ratio
AWFOSR[4:0]).
Each input channel has its own comparator which compares the analog watchdog data
(from analog watchdog filter) with analog watchdog threshold values (AWHT/AWLT). When
several channels are selected (field AWDCH[7:0] field of DFSDM_FLTxCR2 register),
several comparison requests may be received simultaneously. In this case, the channel
request with the lowest number is managed first and then continuing to higher selected
channels. For each channel, the result can be recorded in a separate flag (fields
AWHTF[7:0], AWLTF[7:0] of DFSDM_FLTxAWSR register). Each channel request is
executed in 8 DFSDM clock cycles. So, the bandwidth from each channel is limited to 8
DFSDM clock cycles (if AWDCH[7:0] = 0xFF). Because the maximum input channel
sampling clock frequency is the DFSDM clock frequency divided by 4, the configuration
AWFOSR = 0 (analog watchdog filter is bypassed) cannot be used for analog watchdog
feature at this input clock speed. Therefore user must properly configure the number of
watched channels and analog watchdog filter parameters with respect to input sampling
clock speed and DFSDM frequency.