Analog-to-Digital Converter (ADC)
PXN20 Microcontroller Reference Manual, Rev. 1
34-48 Freescale Semiconductor
34.4.4 Presampling
Presampling allows to precharge or discharge the ADC internal capacitor before it starts
sampling/conversion of the analog input coming from pads. This is useful in the reset information (offset
cancellation) regarding the last converted data. During presampling, the analog ADC samples the
internally generated voltage. During sampling, the analog ADC samples analog input coming from pads.
Presampling can be enabled/disabled on a channel basis by setting the corresponding bits in the PSRn
registers.
After enabling the presampling for a channel, the normal sequence of operation for that channel is
Presampling + Sampling + Conversion. Sampling of the channel can be bypassed by setting the
PRECONV bit in the PSCR register. When sampling of a channel is bypassed, the sampled data of internal
voltage in the presampling state is converted.
Figure 34-45. Presampling Sequence
Figure 34-46. Presampling Sequence with PRECONV = 1
Figure 34-47. Presampling Sequence with PREONCE = 0
Figure 34-48. Presampling Sequence with PRECONV = 1 and PREONCE = 0
Sample B Convert B
Presample C
Sample C Convert C Convert DSample D Sample E
Presample D
Presampling is enabled in Channels C and D. For Channel B, the total clock conversion cycles = (S) + (C).
For Channels C and D, the total clock conversion cycles = (P) + (S) + (C).
Sample B Convert B
Presample C
Convert C
Presample D
Convert ESample E
Convert D
Presampling is enabled in Channels C and D, but sampling is bypassed in these channels by setting
PRECONV = 1 in the PREREG register. For Channels C and D, the total clock conversion cycles = (P) + (C).
Sample B Convert B
Presample C
Sample C
Convert C
Sample E
Convert
D
Sample
D
Presampling is enabled in Channels C and D, but Presampling is done once,
as PREONCE = 0 in the PREREG register.
Sample B Convert B
Presample C Convert
C Sample
D Convert
ESample E
Convert D
Presampling is enabled in Channels C and D. PREONCE = 0 and PRECONV = 1 in the PREREG register.
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