SWRU626 December 2025 CC3501E , CC3551E
The full-scale value output FS by the SINC x digital filter is dependent on the oversampling ratio OSR and is given by:
| OSR | Sinc filter order | |||
|---|---|---|---|---|
| 1 | 2 | 3 | 4 | |
| 4 | 2 | 4 | 6 | 8 |
| 8 | 3 | 6 | 9 | 12 |
| 32 | 5 | 10 | 15 | 20 |
| 64 | 6 | 12 | 18 | 24 |
| 128 | 7 | 14 | 21 | Not Applicable |
| 256 | 6 | 16 | 24 | Not Applicable |
For example, the OSR of 256 with a SINC filter order of 3 results in a full-scale value of 224 or 100:0000h; that is, a value that can be represented by 24 bits. All 24 bits can be accessed using the PDMx.DATAx registers. Depending on the selected data format and alignment, the representation within the PDMx.DATAx registers differs.
With mapping the filter output into the FIFO access mode of 8, 16 and 24 Bits the below max OSR and Filter Order should be selected for the corresponding access mode.
| Data Size [bits] | Data Format: Offset | Data Format: Two's-complement | ||||||
|---|---|---|---|---|---|---|---|---|
| Order | Order | |||||||
| 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | |
| 8 | 256 | 16 | 6 | 4 | 128 | 11 | 5 | 3 |
| 16 | 256 | 256 | 40 | 16 | 256 | 181 | 32 | 13 |
| 24 | 256 | 256 | 256 | 64 | 256 | 256 | 203 | 53 |
Filter should be implemented with 32 bit registers for accumulators and differentiators to increase the accuracy during calculation and to avoid losing bits with rounding effects.