FACT002 [ETC]

Fact 3 Documentation ; 其实3文档\n


型号：	FACT002
厂家：	ETC
描述：	Fact 3 Documentation 其实3文档\n
文件：	总4页 (文件大小：52K)
中文：	中文翻译
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CARE AND FEEDING OF THE PIC16C74

AND ITS PERIPHERALS

By: Robert Angelo

The PIC16C74 is one of the latest mid-range

microcontrollers from Microchip Technology Inc. In this

article we will be addressing a few of the new features and

peripherals of this new part. The main focus will be on the

A/D (Analog-to-Digital) Converter, the SCI-Serial

Communication Interface, and the PWM-Pulse Width

Modulator. Our intention is to give you a small program that

initializes these peripherals as well as exercise them. A

schematic is provided. The PICDEM-2 board from

Microchip will run this program. The second trimpot does

not exist on the PICDEM-2 board, so the second A/D value

may ﬂoat around. The second trimpot is only used to show

a method of changing A/D input pins. If you are using the

PICDEM-2 board, then the LED and a current limiting

resistor must be connected to the PWM output. When the

program is run, the RS-232 terminal will display two A/D

values. The brightness of an LED is adjusted using pulse

width modulation. The duty cycle is determined by the

trimpot setting.

A/D Converter Mysteries

The A/D converter and its eight input channels will be our

ﬁrst topic. Setting up the A/D converter involves two special

function registers:

• ADCON0

• ADCON1

In the program included with this article is a code segment

initad that sets up the A/D. ADCON0 is the work horse

conversion clock frequency and channel. This register is

also where we signal the start of a conversion and detect

the completion of a conversion. ADCON1 has only one

purpose in life for this part, and that is A/D port

conﬁguration. When ADCON1 is used it does not override

the TRISA register controls. The TRISA register must be set

up. Once these registers are set up, all the program has to

do is select the desired pin and set the GO/DONE bit in

ADCON0. The program then waits for the conversion

complete bit, GO/DONE, to be cleared by hardware. Then

the ADRES (A/D conversion result register) register is read.

The value from the ﬁrst pot's conversion is then used to

adjust the PWM pulse width thereby adjusting the LED

brightness.

Assumptions

Although dangerous, sometimes we need to make

assumptions. For this discussion on the PIC16C74, let us

agree that RA0 and RA1 will be connected through a series

resistor to the wipers on two potentiometers with the other

ends connecting across VDD and ground (see schematic).

The oscillator clock will be 4 MHz. First we'll read an A/D

input, send its result out the serial port (to be displayed on

a PC terminal program), and then switch to the next

channel. We will adjust the PWM output pulse width to

match the ﬁrst potentiometer. Each time we are ready to

begin a new sequence we will ﬁrst send a pair of sync bytes

to signal the receiving processor. To simplify our discussion,

we will forgo using interrupts and we will do this in a polled

fashion. The watch dog timer is disabled for this program.

FIGURE 1:

PWM PULSE WIDTH

Period

To ensure there are no surprises, it is a good idea to

initialize every special function register (SFR) and data

Duty

1996 Microchip Technology Inc.

FACT003-page 1

This document was created with FrameMaker 4 0 4

Pulse Width Modulation (PWM)

FIGURE 2:

SERIAL COMMUNICATIONS

INTERFACE MODULE

The PORTC-1 pin is used as the PWM output. The registers

that need to be set up for this PWM operation are:

• TRISC

• T2CON

• CCPR2L

• PR2

• CCP2CON.

The code initpwm is an example of what might be done

to initialize the PWM module. TRISC was cleared earlier

thus setting PORTC as output. By writing a "4" to the

T2CON register, we will set the prescaler equal to 0 and

select TIMER2 operation. Writing a 0fh to the CCP2CON

0fh written to the CCPR2L register sets the high period to a

low value initially. Setting the PR2 register to ffh allows the

CCPR2L value (from the A/D converter result) to approach

a 100% duty cycle. Now we can control the brightness of the

LED attached to this pin by adjusting the pot on pin RA0 and

writing the A/D result to the CCPR2L register as already

described earlier.

First global interrupts are disabled. The initsci code

does the serial port setup and the sendat code handles the

actual sending of the data.

The SCI is setup for 2400 baud, 8 data bits and 1 stop bit

with no parity. A terminal program, such as TERMINAL in

Windows“, set to the same settings can be used to see our

output. If you use the Windows terminal program, then set

the communications parameters to 2400 baud, 8 data bits,

1 stop bit, no parity and hardware handshake.

Tying The Pieces Together

SCI

The main loop for getting the process running and restarting

it again is mloop. The adcnvrt routine handles port pin

selection and actual conversion control. The dopwm routine

handles updating the PWM duty cycle register CCPR2L.

The routine sendatchecks transmit ready status and loads

the transmit buffer when the status reports ready. You will

notice there is no error recovery routine. It is up to the user

to determine.

The Serial Communications Interface Module is our RS-232

communications channel. We will conﬁgure the SCI as an

asynchronous full duplex serial port. This is done with the

routine at initsci in the program provided. There are a

few ﬁne points to remember relative to this peripheral. The

baud rate is determined by a dedicated eight bit baud rate

generator and can be used to derive standard baud rate

frequencies from the oscillator. Since we are not using

interrupts, there are only ﬁve registers to deal with:

Here is what the program will do:

Once all peripherals have been initialized two sync bytes

"< >" are sent to the terminal. The A/D conversion results

are then sent and the LED brightness is adjusted to match

the RA0 trimpot setting. To simplify displaying A/D values,

only the highest nibble is used, and thirty is added to put it

into an ASCII range.

• RCSTA - receiver status

• TXSTA - transmitter status

• TXREG - transmit buffer

• RCREG - receive buffer

• SPBRG - to set the baud rate generator

FACT003-page 2

1996 Microchip Technology Inc.

Care and Feeding of PIC16C74 Source Code

Note: This source code can be downloaded from our BBS from the "PICTIPS" library as ﬁlenameC74feed.ZIP.

LIST P=16C74

INCLUDEP16CXX.INC

;new include file that comes with MPASM (on BBS)

;a/d converter pin count register

;a/d converter pin work register

Adcnt equ

Adcntw equ

20h

21h

Temp

equ

22h

;temporary data holding register seems we always need one

org

goto

org

bcf

init

;go to where our code really begins

;begin program above interrupt service vector address

;make sure we don't get interrupted

init

INTCON,7

PORTA

PORTB

PORTC

PORTD

PORTE

Adcnt

Adcntw

Temp

STATUS,RP0

TRISB

TRISC

TRISD

TRISE

clrf

bsf

clrf

;don't rely on anything, set port latches where you want them

;clear RAM registers we will be using

;switch to page 1 to access trisX registers

;set all ports outputs

;just for this program to minimize current

; and prevent pins from floating

movlw 0Bh

movwf TRISA

;set analog inputs as inputs, the rest as outputs

;

bcf

STATUS,RP0

initad movlw 0C1h

movwf ADCON0

;Internal RC A/D clock, input channel 0 , A/D on

;(user must wait for specified period before sampling)

;select page 1 of the SFRs

bsf

STATUS,RP0

movlw 4

movwf ADCON1

;setup a/d inputs on RA0, RA1 and RA3 with Vref = Vdd

;we are still in page 1 of the SFRs

;setup 2400 baud

initscimovlw

19h

movwf SPBRG

movlw 20h

;setup for async operations

movwf TXSTA

bcf

STATUS,RP0

;back to page 0 for a moment

movlw 80h

movwf RCSTA

;enable serial port operations and the associated pins

clrf

TXREG

RCREG

;clear our serial port buffers for start up

;setup T2CON with prescaler = 0 and timer2 on

;setup capture/compare to PWM mode standard resolution

;set compare register to half for now

initpwmmovlw 4h

movwf T2CON

movlw 0fh

movwf CCP2CON

movlw 0fh

movwf CCPR2L

bsf

STATUS,RP0

;select page 1 for the PR2 register

movlw 0ffh

movwf PR2

bcf

STATUS,RP0

mloop movlw 0dh

;send a carriage return character

call

sendat

movlw 3ch

;begin main loop for data gathering and serial transmission

;these are our sync bytes to tell receiving micro a new

;sequence is beginning

call

sendat

movlw 3eh

call

clrf

adloop call

movf

sendat

Adcnt

adcnvrt

Adcnt,0

;our first time through select AN0 pin

;go do a conversion and send the result

;get Adcnt into the W register

xorlw 2

btfss STATUS,2

;(# determines number of AD inputs to scan)

;have we sampled all of the pins yet?

;go adjust the PWM output

goto

dopwm

mloop

;all done go do it again

1996 Microchip Technology Inc.

FACT003-page 3

adcnvrtmovf

Adcnt,0

;get a/d count value

movwf Adcntw

;put in work register

bcf

rlf

STATUS,0

Adcntw,1

;clear the carry flag for the upcoming rotate operations

;rotate left and leave the number in adcntw

;need to do it three times to put the count in the right

;position to select the next A/D pin

;load the initial ADCON0 value excepting channel select

;set the pin select bits we want

;set the new ADCON0 with new channels selected

;wait about twenty micro seconds

;start conversion

movlw 0C1h

iorwf Adcntw,0

movwf ADCON0

call

bsf

incf

wait

ADCON0,2

Adcnt

;increment pin counter register

adwait btfsc ADCON0,2

goto adwait

;wait for conversion done

;not done yet

swapf ADRES,0

andlw 0Fh

addlw 30h

;conversion done, swap result nibbles into W register

;mask off the upper nibble to limit number to an ascii range

;convert to ascii character to make it visible on terminal

;select page one

sendat bsf

STATUS,RP0

btfss TXSTA,1

;check transmit status ready to send

;if not ready go try again

;back to page 0

goto

bcf

sendat

STATUS,RP0

movwf TXREG

return

;transmit buffer empty send new data

dopwm movf

movwf CCPR2L

goto adloop

ADRES,0

;get the a/d conversion value

;put the value into the PWM duty cycle register

wait

movlw 08h

movwf Temp

decfsz Temp

;do a wait loop of before using a/d converter

goto

return

end

;end of program

FIGURE 3:

PIC16C74 DEMO SCHEMATIC

FACT003-page 4

1996 Microchip Technology Inc.

FACT002 [ETC]

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