DN_030.pdf

D ESIGN N OTE

#030

A UTHOR :

K EYWORDS :

D HANANJAY V. G ADRE - DVGADRE @ NETKRACKER . COM

UART, F REQUENCY S YNTHESIZER

This document is originally distributed by AVRfreaks.net, and may be distributed, reproduced, and modified

without restrictions. Updates and additional design notes can be found at: www.AVRfreaks.net

Frequency Synthesizer

Introduction

Frequency Synthesizer using the concept of Numeric Controlled Oscillator. The Synthe-

sizer is programmed through the serial port. An effective synthesizer for frequencies in

the range of milli-Hz to 10s of KHz.

Can be used with Any AVR with a serial port uses a 40-bit summer value of the 40 bits is

received from the serial port at 9600-8-N-1 ASCII encoded format: 123456789A Left to

right --> MSB to LSB

Using the

Synthesizer

For an 8MHz clock, and a 40-bit Adder as used in this Program, the user is required to

send a data string from the PC in 9600-8-N-1 format. The string starts with “ * ” followed

by a 10-digit number in ASCII. The 10 digit number “N” is calculated as follows:

N = (2^40)*F*10^(-6) (i.e., N = 1099511.627776 * F). F is the required frequency and

10^(-6) is the cycle time for the adder loop.

Example: Lets assume you want 1KHz frequency. Therefore N = 004189374B. So

transmit the following string to the AVR through the PC serial port: *004189374B and the

AVR will generate 1 KHz signal on the PB7 pin. Similarly, to generate a 225 Hz signal,

N = 000EBEDFA4. So the string is: *000EBEDFA4.

Code

Use the appropriate path of the 2313def.inc file on your system

;******************************************

.include "c:\avrtools\appnotes\2313def.inc"

;******************************************

.equ baudrate=$33 ; 9600 at 8MHz

.def sundry=r16

.def sum0=r20

.def sum1=r21

.def sum2=r22

.def sum3=r23

.def sum4=r24

.def rreg=r25

.def err_code=r26

.def rtemp=r16

.def f0=r1

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Design Note #030 – Date: 06/02

.def f1=r2

.def f2=r3

.def f3=r4

.def f4=r5

.cseg

.org 0

rjmp STARTUP

;*********************************

;Receive data Interrupt Subroutine

.org 7

rjmp get_data

;*********************************

.org 20

STARTUP:

ldi sundry, low(RAMEND) ; Stack pointer init.

out SPL, sundry

ldi sundry, 255

;Output port for PORTB.

out DDRB, sundry

;load default value for the NCO

ldi sundry, $9c

mov f0, sundry

ldi sundry, $dc

mov f1, sundry

ldi sundry, $ef

mov f2, sundry

ldi sundry, $50

mov f3, sundry

ldi sundry, $00

mov f4, sundry

rcall init_uart

sei

;**************************************************

;Main Frequency Synthesizing Loop

loop: add sum0, f0

adc sum1, f1

adc sum2, f2

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Design Note #030 – Date: 06/02

adc sum3, f3

adc sum4, f4

out PORTB, sum4

; Output Frequency on PB7

rjmp loop

;**************************************************

;Initialize the UART for 9600-8-N-1 Data Format

init_uart:

ldi rtemp, baudrate ;setting baud rate

out UBRR,rtemp

ldi rtemp, $98 ;initialising UART control register

;RXCIE interrupt is enabled.

out UCR, rtemp

ret

;***************************************************

;Serial Data Receive ISR

;Routine is invoked when a byte is received.

;The Routine then receives 7 more bytes, if the first

;byte is ’*’. Else it returns back to the main program

get_data:

in r0, sreg

in sundry, ucr

;disable further receive interrupt

andi sundry, $7f

out ucr, sundry

rxcomp: sbis USR,RXC

;poll to check if char received

rjmp rxcomp

in rreg,UDR

;put received data in rreg

out UDR, rreg

;echo the character

cpi rreg, 42

;start of data header is: ’*’

brne go_back

; If not, GO BACK.

ldi err_code, 0

rcall rx_byte

cpi err_code, 0

brne go_back

mov f4, sundry

rcall rx_byte

cpi err_code, 0

brne go_back

mov f3, sundry

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Design Note #030 – Date: 06/02

rcall rx_byte

cpi err_code, 0

brne go_back

mov f2, sundry

rcall rx_byte

cpi err_code, 0

brne go_back

mov f1, sundry

rcall rx_byte

cpi err_code, 0

brne go_back

mov f0, sundry

clr sum0

clr sum1

clr sum2

clr sum3

clr sum4

go_back:

in sundry, ucr

ori sundry, $80

;enable receive complete interrupt again

out ucr, sundry

out sreg, r0

reti

rx_byte:

sbis USR, 7

;poll to check if char received

rjmp rx_byte

in rreg, UDR

out UDR, rreg

cpi rreg,$40

brsh asnxt

subi rreg,$30

rjmp asnxt1

asnxt: subi rreg,$37

asnxt1: mov sundry, rreg

swap sundry

andi sundry, $f0

rx_byte1:

sbis USR, 7

;poll to check if char received

rjmp rx_byte1

in rreg, UDR

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Design Note #030 – Date: 06/02

out UDR, rreg

cpi rreg,$40

brsh asnxt2

subi rreg,$30

rjmp asnxt3

asnxt2: subi rreg,$37

asnxt3: andi rreg, $0f

or sundry, rreg

ret

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Design Note #030 – Date: 06/02

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