SC312XDP512F0MPVR datasheet(138/1350 Pages) FREESCALE

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Chapter 4 Analog-to-Digital Converter (ATD10B16CV4) Block Description

MC9S12XDP512 Data Sheet, Rev. 2.17

138

Freescale Semiconductor

4.3.2.5

ATD Control Register 4 (ATDCTL4)

This register selects the conversion clock frequency, the length of the second phase of the sample time and

the resolution of the A/D conversion (i.e., 8-bits or 10-bits). Writes to this register will abort current

conversion sequence but will not start a new sequence.

Read: Anytime

Write: Anytime

76543210

SRES8

SMP1

SMP0

PRS4

PRS3

PRS2

PRS1

PRS0

Reset

00000101

Figure 4-7. ATD Control Register 4 (ATDCTL4)

Table 4-11. ATDCTL4 Field Descriptions

Field

Description

SRES8

A/D Resolution Select — This bit selects the resolution of A/D conversion results as either 8 or 10 bits. The

A/D converter has an accuracy of 10 bits. However, if low resolution is required, the conversion can be speeded

up by selecting 8-bit resolution.

0 10 bit resolution

1 8 bit resolution

6:5

SMP[1:0]

Sample Time Select —These two bits select the length of the second phase of the sample time in units of ATD

conversion clock cycles. Note that the ATD conversion clock period is itself a function of the prescaler value

(bits PRS4-0). The sample time consists of two phases. The ﬁrst phase is two ATD conversion clock cycles

long and transfers the sample quickly (via the buffer ampliﬁer) onto the A/D machine’s storage node. The

second phase attaches the external analog signal directly to the storage node for ﬁnal charging and high

accuracy. Table 4-12 lists the lengths available for the second sample phase.

4:0

PRS[4:0]

ATD Clock Prescaler — These 5 bits are the binary value prescaler value PRS. The ATD conversion clock

frequency is calculated as follows:

Note: The maximum ATD conversion clock frequency is half the bus clock. The default (after reset) prescaler

value is 5 which results in a default ATD conversion clock frequency that is bus clock divided by 12.

Table 4-13 illustrates the divide-by operation and the appropriate range of the bus clock.

Table 4-12. Sample Time Select

SMP1

SMP0

Length of 2nd Phase of Sample Time

2 A/D conversion clock periods

4 A/D conversion clock periods

8 A/D conversion clock periods

16 A/D conversion clock periods

ATDclock

BusClock

[]

PRS

[]

--------------------------------

0.5

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