PIC16F74-ESLASHML datasheet(89/174 Pages) MICROCHIP

89 / 174 page

 2002 Microchip Technology Inc.

DS30325B-page 87

PIC16F7X

11.2

Selecting the A/D Conversion

Clock

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

A/D conversion requires 9.0 TAD per 8-bit conversion.

The source of the A/D conversion clock is software

selectable. The four possible options for TAD are:

• 2 TOSC (FOSC/2)

• 8 TOSC (FOSC/8)

• 32 TOSC (FOSC/32)

• Internal RC oscillator (2-6

µs)

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

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

as small as possible, but no less than 1.6

µs.

11.3

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 correspond-

ing TRIS bits set (input). If the TRIS bit is cleared (out-

put), the digital output level (VOH or VOL) will be

converted.

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

CHS2:CHS0 bits and the TRIS bits.

11.4

A/D Conversions

Setting the GO/DONE bit begins an A/D conversion.

When the conversion completes, the 8-bit result is

placed in the ADRES register, the GO/DONE bit is

cleared, and the ADIF flag (PIR<6>) is set.

If both the A/D interrupt bit ADIE (PIE1<6>) and the

peripheral interrupt enable bit PEIE (INTCON<6>) are

set, the device will wake from SLEEP whenever ADIF

is set by hardware. In addition, an interrupt will also

occur if the global interrupt bit GIE (INTCON<7>) is set.

Clearing the GO/DONE bit during a conversion will

abort the current conversion. The ADRES register will

NOT be changed, and the ADIF flag will not be set.

After the GO/DONE bit is cleared at either the end of a

conversion, or by firmware, another conversion can be

initiated by setting the GO/DONE bit. Users must still

take into account the appropriate acquisition time for

the application.

11.5

A/D Operation During SLEEP

The A/D module can operate during SLEEP mode. This

requires that the A/D clock source be set to RC

(ADCS1:ADCS0 = ‘11’). When the RC clock source is

selected, the A/D module waits one instruction cycle

before starting the conversion. This allows the SLEEP

instruction to be executed, which eliminates all digital

switching noise from the conversion. When the conver-

sion is completed, the GO/DONE bit will be cleared,

and the result loaded into the ADRES register. If the

A/D interrupt is enabled, the device will wake-up from

SLEEP. If the A/D interrupt is not enabled, the A/D mod-

ule will then be turned off, although the ADON bit will

remain set.

When the A/D clock source is another clock option (not

RC), a SLEEP instruction will cause the present conver-

sion to be aborted and the A/D module to be turned off,

though the ADON bit will remain set.

Turning off the A/D places the A/D module in its lowest

current consumption state.

11.6

Effects of a RESET

A device RESET forces all registers to their RESET

state. The A/D module is disabled and any conversion

in progress is aborted. All A/D input pins are configured

as analog inputs.

The ADRES register will contain unknown data after a

Power-on Reset.

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 digital input buffer to con-

sume current that is out of the device’s

specification.

Note:

The GO/DONE bit should NOT be set in

the same instruction that turns on the A/D.

Note:

For the A/D module to operate in SLEEP,

the A/D clock source must be set to RC

(ADCS1:ADCS0 = 11). To perform an A/D

conversion in SLEEP, ensure the SLEEP

instruction immediately follows the instruc-

tion that sets the GO/DONE bit.

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