PIC12C67X datasheet(49/129 Pages) MICROCHIP

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DS30561B-page 49

PIC12C67X

8.2

Selecting the A/D Conversion Clock

The A/D conversion time per bit is defined as TAD. The

A/D conversion requires 9.5 TAD per 8-bit conversion.

The source of the A/D conversion clock is software

selected. The four possible options for TAD are:

•2TOSC

•8TOSC

•32TOSC

• Internal ADC RC oscillator

For correct A/D conversions, the A/D conversion clock

(TAD) must be selected to ensure a minimum TAD time

of 1.6

µs. If the minimum TAD time of 1.6 µs can not be

obtained, TAD should be

≤8 µs for preferred operation.

Table 8-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

8.3

Configuring Analog Port Pins

The ADCON1 and TRIS Registers control the opera-

tion of the A/D port pins. The port pins that are desired

as analog inputs must have their corresponding TRIS

bits set (input). If the TRIS bit is cleared (output), the

digital output level (VOH or VOL) will be converted.

The A/D operation is independent of the state of the

CHS<2:0> bits and the TRIS bits.

Note 1: When reading the port register, all pins

configured as analog input channel will

read as cleared (a low level). Pins config-

ured as digital inputs, will convert an ana-

log input. Analog levels on a digitally

configured input will not affect the conver-

sion accuracy.

2: Analog levels on any pin that is defined as

a digital input (including the AN<3:0>

pins) may cause the input buffer to con-

sume current that is out of the devices

specification.

TABLE 8-1:

TAD vs. DEVICE OPERATING FREQUENCIES

AD Clock Source (TAD)

Device Frequency

Operation

ADCS<1:0>

4 MHz

1.25 MHz

333.33 kHz

2TOSC

500 ns(2)

1.6

µs6 µs

8TOSC

2.0

µs6.4 µs

µs(3)

32TOSC

8.0

µs

25.6

µs(3)

Internal ADC RC Oscillator(5)

2 - 6

µs(1,4)

2 - 6

µs(1,4)

2 - 6

µs(1)

Note 1:

The RC source has a typical TAD time of 4

µs.

2: These values violate the minimum required TAD time.

3: For faster conversion times, the selection of another clock source is recommended.

4: While in RC mode, with device frequency above 1 MHz, conversion accuracy is out of specification.

5: For extended voltage devices (LC), please refer to Electrical Specifications section.

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