DM164120-1 Datasheet DM164120-1 | P25-P27

LPC Demo Board Lessons

3.2.4.2 ADCON0

ADCON0 controls the ADC operation. Bit 0 turns on the ADC module. Bit 1 starts a

conversion and bits <5:2> selects which channel the ADC will operate. VCFG bit< 6>

selects the ADC reference, which may be either V

DD or a separate reference voltage

on V

REF. ADFM bit <7> selects whether the 10 bits are right or left justified in the 16 bits.

For purposes of this lesson, the ADC must be turned on and pointed to RA0. Choose

the internal voltage reference and 8T

OSC conversion clock.

The ADC needs about 5 μs, after changing channels, to allow the ADC sampling

capacitor to settle. Finally, we can start the conversion by setting the GO bit in ADCON0.

The bit also serves as the DONE

flag. That is, the ADC will clear the same bit when the

conversion is complete. The answer is then available in ADRESH:ADRESL.

This lesson takes the high order 4 bits of the result and copies them to the display LEDs

attached to PORTC.

See the Analog-to-Digital section in the PIC16F685/687/689/690 Data Sheet

(DS41262) for more details on the ADC module.

R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0

ADFM VCFG CHS3 CHS2 CHS1 CHS0 GO/DONE

ADON

bit 7 bit 0

bit 7 ADFM: A/D Result Formed Select bit

1 = Right justified

0 = Left justified

bit 6 VCFG: Voltage Reference bit

1 = V

REF pin

0 = V

bit 5-2 CHS<3:0>: Analog Channel Select bits

0000 = Channel 00 (AN0)

0001 = Channel 01 (AN1)

0010 = Channel 02 (AN2)

0011 = Channel 03 (AN3)

0100 = Channel 04 (AN4)

0101 = Channel 05 (AN5)

0110 = Channel 06 (AN6)

0111 = Channel 07 (AN7)

1000 = Channel 08 (AN8)

1001 = Channel 09 (AN9)

1010 = Channel 10 (AN10)

1011 = Channel 11 (AN11)

1100 =CV

REF

1101 =VP6

1110 = Reserved. Do not use.

1111 = Reserved. Do not use.

bit 1 GO/DONE

: A/D Conversion Status bit

1 = A/D conversion cycle in progress. Setting this bit starts an A/D conversion cycle.

This bit is automatically cleared by hardware when the A/D conversion has completed.

0 = A/D conversion completed/not in progress

bit 0 ADON: A/D Enable bit

1 = A/D converter module is enabled

0 = A/D converter is shut off and consumes no operating current

Legend:

R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’

- n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown

Low Pin Count Demo Board User’s Guide

3.2.5 Lesson 5: Variable Speed Rotate

Lesson 5 combines Lessons 3 and 4 by using the Analog-to-Digital Converter (ADC)

to control the speed of rotation.

New Instructions

BTFSS Bit test, skip if set

BTFSC Bit test, skip if clear

A conversion is run on every pass through the main loop. The result controls the length

of the outer loop (see Example 3-5).

EXAMPLE 3-5: VARIABLE SPEED ROTATE EXAMPLE

FIGURE 3-3: VARIABLE SPEED ROTATE PROGRAM FLOW

...

BSF ADCON0,GO ;start conversion

BTFSS ADCON0,GO ;this bit will change to zero when the

;conversion is complete

GOTO $-1

MOVF ADRESH,w ;Copy the display to the LEDs

ADDLW 1

MOVWF Delay2

A2DDelayLoop

DECFSZ Delay1,f ;Delay Loop shortened by the ADResult as

;controlled by the Pot.

GOTO A2DDelayLoop

DECFSZ Delay2,f

GOTO A2DDelayLoop

Initialize I/O Port

Put Up Display

Delay Using ADC

Rotate Display

Reset Display

Did it overflow?

Yes

Initialize ADC

Get ADC Result

LPC Demo Board Lessons

3.2.6 Lesson 6: Switch Debouncing

Mechanical switches play an important and extensive role in practically every

computer, microprocessor and microcontroller application. Mechanical switches are

inexpensive, simple and reliable. In addition, switches can be very noisy. The apparent

noise is caused by the closing and opening action that seldom results in a clean

electrical transition. The connection makes and breaks several, perhaps even

hundreds, of times before the final switch state settles.

The problem is known as switch bounce. Some of the intermittent activity is due to the

switch contacts actually bouncing off each other. Imagine slapping two billiard balls

together. The hard non-resilient material doesn’t absorb the kinetic energy of motion.

Instead, the energy dissipates over time and friction in the bouncing action against the

forces push the billiard balls together. Hard metal switch contacts react in much the

same way. Also, switch contacts are not perfectly smooth. As the contacts move

against each other, the imperfections and impurities on the surfaces cause the

electrical connection to be interrupted. The result is switch bounce.

The consequences of uncorrected switch bounce can range from being just annoying

to catastrophic. For example, imagine advancing the TV channel, but instead of getting

the next channel, the selection skips one or two. This is a situation a designer should

strive to avoid.

Switch bounce has been a problem even before the earliest computers. The classic

solution involved filtering, such as through a resistor-capacitor circuit, or through

re-settable shift registers (see Figures 3-4 and 3-5). These methods are still effective

but they involve additional cost in material, installation and board real estate. Why

suffer the additional expense when software is free and program memory is abundant.

FIGURE 3-4: FILTERING DEBOUNCE SOLUTION

FIGURE 3-5: SHIFT REGISTER DEBOUNCE SOLUTION

Filtered

Switch

Output

Filtered

Switch

Output

Debounce

Clock

CLK

CLR

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P33 P34-P36 P37-P39 P40-P42

DM164120-1

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