Freescale Semiconductor MPC5565, Reference Manual

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Enhanced Queued Analog-to-Digital Converter (eQADC)
MPC5565 Microcontroller Reference Manual, Rev. 1.0
18-88 Freescale Semiconductor
18.4.5.2 ADC Clock and Conversion Speed
The clock input to the ADCs is defined by setting the ADC0_CR[ADC0_CLK_PS] and
ADC1_CR[ADC1_CLK_PS] fields. Refer to
Section 18.3.3.1, “ADCn Control Registers (ADC0_CR and
ADC1_CR) .” The ADC0/1_CLK_PS field selects the clock divide factor by which the system clock is
divided as showed in Table 18-28 . The ADC clock frequency is calculated as below and it must not exceed
12 MHz.
Figure 18-48 depicts how the ADC clocks for ADC0 and ADC1 are generated.
Figure 18-48. ADC0/1 Clock Generation
The ADC conversion speed (in kilosamples per second – ksamp/s) is calculated by the following formula.
The number of sampling cycles is determined by the LST bits in the command message — see
Section ,
“Conversion Command Message Format for On-Chip ADC Operation ,” — and it can take one of the
following values: 2, 8, 64, or 128 ADC clock cycles. The number of AD conversion cycles is 13 for
differential conversions and 14 for single-ended conversions. The maximum conversion speed is achieved
when the ADC Clock frequency is set to its maximum (12Mhz) and the number of sampling cycles set to
its minimum (2 cycles). The maximum conversion speed for differential and single-ended conversions are
800ksamp/s and 750ksamp/s, respectively.
Table 18-47 shows an example of how the ADC0/1_CLK_PS can be set when using a 120 MHz system
clock and the corresponding conversion speeds for all possible ADC clock frequencies. The table also
shows that according to the system clock frequency, certain clock divide factors are invalid (2, 4, 6, 8 clock
divide factors in the example) since their use would result in a ADC clock frequency higher than the
maximum one supported by the ADC. ADC clock frequency must not exceed 12 Mhz.
ADCClockFrequency
SystemClockFrequency MHz
( )
SystemClockDivideFactor
---------------------------------------------------------------------------------------- ADCClockFrequency 12MHz
≤ ( )
; =
Divide by:
2, 4, 6, ..., 60, 62, 64
ADC0
control register
To ADC0
ADC0_CLK_PS
ADC0 clock
System
clock
System clock divider
Divide by:
2, 4, 6, ..., 60, 62, 64
ADC1
control register
To ADC1
ADC1_CLK_PS
ADC1 Clock
System
clock
System Clock Divider
ADCConversionSpeed
ADCClockFrequency MHz
( )
NumberOfSamplingCycles NumberOfADConversionCycles +
( )
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- =