Conversion Clock

The conversion clock source is selected with the CS bit. When CS = 1, the ADC clock source is an internal fixed-frequency clock referred to as ADCRC. When CS = 0, the ADC clock source is derived from FOSC.

Important: When CS = 0, the clock can be divided using the ADCLK register to meet the ADC clock period requirements.

The time to complete one bit conversion is defined as the TAD. Refer to Figure 1 for the complete timing details of the ADC conversion.

For correct conversion, the appropriate TAD specification must be met. Refer to the ADC Timing Specifications table in the “Electrical Specifications” chapter of the device data sheet for more details. The table below gives examples of appropriate ADC clock selections.

Table 1. ADC Clock Period (TAD) vs. Device Operating Frequencies(3,5)
ADC Clock Source ADCLK(2) ADC Clock Period (TAD) for Different Device Frequency (FOSC)
32 MHz(1) 20 MHz(1) 16 MHz(1) 8 MHz 4 MHz 1 MHz
FOSC/2 ‘b000000(2) 62.5 ns(4) 100 ns(4) 125 ns(4) 250 ns(4) 500 ns(4) 2.0 μs
FOSC/4 ‘b000001 125 ns(4) 200 ns(4) 250 ns(4) 500 ns(4) 1.0 μs 4.0 μs
FOSC/6 ‘b000010(2) 187.5 ns(4) 300 ns(4) 375 ns(4) 750 ns(4) 1.5 μs 6.0 μs
FOSC/8 ‘b000011 250 ns(4) 400 ns(4) 500 ns(4) 1.0 μs 2.0 μs 8.0 μs
... ... ... ... ... ... ... ...
FOSC/16 ‘b000111 500 ns(4) 800 ns(4) 1.0 μs 2.0 μs 4.0 μs 16.0 μs(4)
... ... ... ... ... ... ... ...
FOSC/32 ‘b001111 1.0 μs 1.6 μs 2.0 μs 4.0 μs 8.0 μs 32.0 μs(4)
... ... ... ... ... ... ... ...
FOSC/64 ‘b0111111 2.0 μs 3.2 μs 4.0 μs 8.0 μs 16.0 μs(4) 64.0 μs(4)
... ... ... ... ... ... ... ...
FOSC/128 ‘b111111 4.0 μs 6.4 μs 8.0 μs 16.0 μs(4) 32.0 μs(4) 128.0 μs(4)
ADCRC CS = 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
Notes:
  1. 1.Frequencies above 8 MHz are not recommended when using FOSC as the clock source.
  2. 2.For accurate ADC results, loading the ADCLK register with even-numbered values (e.g. 0x00, 0x02, 0x04, etc.) is not recommended.
  3. 3.Refer to the "Electrical Specifications" chapter of the device data sheet to see the TAD parameter for the ADCRC source typical TAD value.
  4. 4.These values violate the required TAD time.
  5. 5.The ADC clock period (TAD) and total ADC conversion time can be minimized when the ADC clock is derived from the system clock FOSC. However, the ADCRC oscillator source must be used when conversions are to be performed with the device in Sleep mode.
Important:
Figure 1. Analog-to-Digital Conversion Cycles