PIC16C765SLASHP datasheet(93/158 Pages) MICROCHIP

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DS41124A-page 93

PIC16C745/765

12.2

Selecting the A/D Conversion Clock

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

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

The source of the A/D conversion clock is software

selectable. The four possible options for TAD are:

•2TOSC

•8TOSC

•32TOSC

• Dedicated Internal 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.

TABLE 12-1:

TAD vs. DEVICE OPERATING FREQUENCIES

12.3

Configuring Analog Port Pins

The ADCON1, TRISA and TRISE registers control the

operation 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.

12.4

A/D Conversions

Clearing the GO/DONE bit during a conversion will

abort the current conversion. The ADRES register will

NOT be updated with the partially completed A/D con-

version sample. That is, the ADRES register will con-

tinue to contain the value of the last completed

conversion (or the last value written to the ADRES reg-

ister). After the A/D conversion is aborted, a 2TAD wait

is required before the next acquisition is started. After

this 2TAD wait, an acquisition is automatically started on

the selected channel.

AD Clock Source (TAD)

Device Frequency

Operation

ADCS1:ADCS0

20 MHz

5 MHz

1.25 MHz

333.33 kHz

2TOSC

100 ns(2)

400 ns(2)

1.6

µs6 µs

8TOSC

400 ns(2)

1.6

µs6.4 µs

µs(3)

32TOSC

1.6

µs6.4 µs

25.6

µs(3)

2 - 6

µs(1,4)

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: For device frequencies above 1 MHz, the device must be in SLEEP for the entire conversion, or the A/D

accuracy may be out of specification.

Note 1: When reading the port register, all pins

configured as analog input channels 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, but not as an analog input,

may cause the input buffer to consume

current that is out of specification.

3: The TRISE register is not provided on the

PIC16C745.

Note:

The GO/DONE bit should NOT be set in

the same instruction that turns on the A/D.

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