PXS20RM Datasheet, PDF (199/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

The minimum sampling phase duration is:
Tsample = 7  Tck
For INPSAMP > 8, the sampling phase duration is:
Tsample = INPSAMP – 1  Tck
The total evaluation phase duration is:
Teval = 12  Tbiteval
The total conversion duration is (not including external multiplexing):
Tconv = Tsample + Teval + T_ck
Analog-to-Digital Converter (ADC)
9.4.5 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 for resetting information
regarding the last converted data. During presampling, the analog ADC samples the internally generated
voltage while in the 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 PSRs.
After enabling the presampling for a channel, normal sequence of operation will be
Presampling+Sampling+Conversion for that channel. Sampling of the channel can be bypassed by setting
the PSCR[PRECONV] bit. When sampling of a channel is bypassed, the sampled data of internal voltage
in the Presampling State will be converted (see Figure 9-46 and Figure 9-47).
Sample B Evaluate B Presample C Sample C
Evaluate C Presample D Sample D Evaluate D Sample E
Presampling is enabled in the channel C and D.
For channel B, the total number of conversion clock cycles = (Sample) + (Convert).
For channel C and D, the total number of conversion clock cycles = (Presample) + (Sample) + (Convert).
Figure 9-46. Presampling sequence
Sample B
Evaluate B
Presample C
Evaluate C
Presample D
Evaluate D
Sample E
Evaluate E
Presampling enabled in channel C and D but sampling is bypassed in these channels by setting PSCR[PRECONV] = 1.
For channel C and D, the total number of conversion clock cycles = (Presample) + (Convert).
Figure 9-47. Presampling sequence with PRECONV = 1
The presampling channels implemented on this device are:
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
9-41