33.1.4 Conversion Clock
The conversion clock source is software selected with the ADCS bit. When ADCS = 1 the ADC clock source is an internal fixed-frequency clock referred to as FRC. When ADCS = 0 the ADC clock frequencies are derived from FOSC. The ADCLK33.7.6 ADCLK register selects one of 64 possible clock options from FOSC/2 to FOSC/128:
- FOSC/2
- FOSC/4
- FOSC/6
- FOSC/8
- FOSC/10
- ...
- FOSC/128
The time to complete one bit conversion is defined as the TAD. One full 10-bit conversion requires 11.5 TAD periods as shown in Figure 33-2.
For correct conversion, the appropriate TAD specification must be met. Refer to the "ADC Timing Specifications" for more information. The "ADC Clock Period" table below gives examples of appropriate ADC clock selections.
- Except for the FRC clock source, any changes in the system clock frequency will change the ADC clock frequency, which may adversely affect the ADC result.
- The internal control logic of the ADC runs off of the clock
selected by ADCS. When the ADCS is set to
‘
1
’ (ADC runs on FRC), there may be unexpected delays in operation when setting ADC control bits.
ADC Clock Period (TAD) | Device Frequency (FOSC) | |||||||
---|---|---|---|---|---|---|---|---|
ADC Clock Source | ADCLK | 64 MHz | 32 MHz | 20 MHz | 16 MHz | 8 MHz | 4 MHz | 1 MHz |
FOSC/2 | 000000 | 31.25 ns(2) | 62.5 ns(2) | 100 ns(2) | 125 ns(2) | 250 ns(2) | 500 ns(2) | 2.0 μs |
FOSC/4 | 000001 | 62.5 ns(2) | 125 ns(2) | 200 ns(2) | 250 ns(2) | 500 ns(2) | 1.0 μs | 4.0 μs |
FOSC/6 | 000010 | 125 ns(2) | 187.5 ns(2) | 300 ns(2) | 375 ns(2) | 750 ns(2) | 1.5 μs | 6.0 μs |
FOSC/8 | 000011 | 187.5 ns(2) | 250 ns(2) | 400 ns(2) | 500 ns(2) | 1.0 μs | 2.0 μs | 8.0 μs(3) |
... | ... | ... | ... | ... | ... | ... | ... | ... |
FOSC/16 | 000111 | 250 ns(2) | 500 ns(2) | 800 ns(2) | 1.0 μs | 2.0 μs | 4.0 μs | 16.0 μs(3) |
... | ... | ... | ... | ... | ... | ... | ... | ... |
FOSC/128 | 111111 | 2.0 μs | 4.0 μs | 6.4 μs | 8.0 μs | 16.0 μs(3) | 32.0 μs(2) | 128.0 μs(2) |
FRC | ADCS=1 | 1.0-6.0 μs | 1.0-6.0 μs | 1.0-6.0 μs | 1.0-6.0 μs | 1.0-6.0 μs | 1.0-6.0 μs | 1.0-6.0 μs |
Note:
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