PIC16F84 Based Morse Code Reader
So you want to become a HAM, or you've got one of those no-code licences, but like me feel somewhat lacking,... not having obtaining that age old badge of proficiency that differientiated the HAM from other radio operators. However, like thousands and thousands of others, you have trouble learning the code. The problem for most people is the non-real time nature of the process, i.e., writing down the last character while listening to, and decoding the signature of the next character. Furthermore, when you make a mistake, the entire process collapses as your mind tries to perform error correction, trying to fill in the missing blanks, causing you to miss even more characters. One way out of this delimma is to remove the burden of writing down the characters altogether during the process of building up your code speed. But to do this you need a device that copies and displays the code in parallel with you, which is what the stand-alone device described in this article is designed to perform. The decoder is designed for code speeds ranging from about 6 words per minute (WPM) to greater than 36 WPM. The rate adaptive algorithm responds quickly to code speed changes, so you can copy both halves of a QSO, even when the parties transmit at different rates. The HardwareThe schematic of the decoder is shown in figures 1a and 1b. It consists of four major pieces, all powered from a set of four (4) AA cells. The first piece, the front-end, is composed of an electret microphone and a common emitter transistor amplifier. This building block provides a wireless hookup to your radio receiver or code practice oscillator. The 15Kohm resistor biasing the electret may have to be changed to a different value, depending on the requirements of the electret you use. In addition to providing gain, the transistor amplifier also acts as a first level bandpass filter. Its band edges are determined by the size of the coupling capacitors, and the feedback capacitor between Q1's base and collector terminals.
Figure 1 b.
 The second functional block is a narrowband PLL based tone detector, consisting of a tunable NE567 PLL tone decoder. There's nothing radical here, the circuit is right out of the manufacturers data sheet, and employs hysteresis for chatter prevention to, in effect, debounce the decoded signal. The small signal, narrowband detection capability of this block enables one to easily discriminate one signal from another, even when the signal you are copying is substantially smaller that the adjacent channel interference, as long as there is 100Hz or so frequency separation between them. The output of the detector is a one-zero pattern replicating the DIT-DAH sequence of the received signal. This output drives both an input to the PIC16F84 microcontroller and an LED which is used as a receiver tuning aid. More on that later. The third functional block is the PIC16F84 microcontroller (CPU). Its function is to measure the duration of the one-zero input string from the tone decoder, and translate the pattern into DITs, DAHs, symbol spaces, character spaces, or word spaces. The CPU also perfoms input signal debounce, just in case the front-end missed something. This feature was one I found absolutely essential for the robust operation of the decoder under varying signal conditions. The CPU also has the task of code speed adaptation, which it performs by performing a running average on the various components of the signal in real time. The symbol averages are then used to compute time threshold levels for correct symbol interpretation. As each of the symbols are received, a "code word" is assembled and used to lookup/convert to its ASCII equivalent character for display. The CPU also drives an LED in synchronism with the input. While this feature was used initially as a debug aid, it also serves as a tuning aid, and verifies that the CPU is receiving what the front-end sent. Finally, the CPU interfaces to the LCD line display, sending ASCII characters to it and monitoring LCD status. The final building block is the LCD display. I used a surplus display that uses an Hitachi HD44780 LCD controller based interface. You can purchase a similiar OPTREX unit through DigiKey Electronics, P/N DCM-117A, for about $12.00, or Hitachi display I used from me for $8.00, while my limited supply lasts. In the prototype implementation of the CPU, figure 1a, I used a crystal to generate the CPU clock since I had a sizeable stash of crystals in my junk box. But one could just as easily use the RC oscillator configuration since the PIC16F84 spends most of its time sitting around anyway, and could easily do the job running at 1MHz. ConstructionI built my decoder as two separate pieces, and recommend this approach for two reasons.1) It allows you to distance the CPU and LCD away from your receiver, minimizing digital noise coupling the the receiver. 2) It keeps the CPU and LCD clock noise away from the sensitive front-end of the electret amplifier and PLL. The front end was constructed using point-to-point wiring on a 2.0" by 2.5" piece of perf-board. I wrapped the edges with adhesive backed copper tape to provide a convenient place to ground components. The 10Kohm variable resistor off pin 5 of the NE567 provides frequency adjustment capability so you can tune it to the frequency of a code practice oscillator, or to a comfortable pitch to copy signals off the airwaves. A three (3) wire interface connects the front-end to the CPU/LCD assembly, carrying the signal, power and ground. The CPU was also point-to-point wired on a 2" by 2.5" perf-board. I decided to interconnect the CPU and LCD using a 14 pin (2 * 7) plug and socket arrangement I made up for easy connect and disconnect. I used machined pin and collet type sockets and pins for this connection. A three pin header socket was used to make the connection to the front-end consisting of power, ground, and the decoder output. All the parts used in this design are readily available through many suppliers. The total cost of the unit is less than $40.00, depending on how you choose to package the assemblies. Subtotals for the functional blocks are roughly: $4.00 for the front-end parts, $12.00 for the LCD display, and $15.00 for a preprogrammed PIC16F84 from the author, and $3.00 for miscellaneous items. If you choose to order a preprogrammed PIC16F84 from me, please specify -HS for the 4.9152MHz crystal controlled version, or -RC for the RC oscillator version. Send $15.00 for the CPU plus $3.00 S&H to: Lawrence Foltzer The PIC16F84 Assembler/Machine CodeThe firmware for this project is shown in Listing 1. This code was written in Microchip's native assemby language, and only took 365 instruction to implement. To help follow the flow, read the pseudocode at the end of the listing. You will find other important design details at the end of the listing also. The heart of the code is two running average buffers that keep tabs on the length of the four previous DIT and DAH interval samples. With these short buffers, speed adaptation appears almost instantanous. But I must confess that I didn't start out with this approach. In my first implemetation, I averaged DITs and DAHs in the same buffer, since over time there would roughly be an equal number of DITs and DAHs, and the average, therefore, would be (2 DITs long) right in the middle and be the perfect symbol discriminator. But when you copy sequences like "he is 55 .....", the symbol decision threshold becomes corrupted quickly unless the buffer depth is quite large. But a larger buffer means a slow code speed adaptation rate, so I quickly abandoned that approach in favor of the dual buffer approach. The three subroutines at the end of the code drive the LCD display interface. Perhaps you will find uses for them in other PIC related projects. DIT DAH DIT DIT DAH DAH DAH DAH DAH DAH DAH DIT DAH DAH ! DAH DIT DAH DIT DAH DIT DIT DIT DIT DIT DIT DAH DAH DIT DIT DIT DAH DIT DIT DAH DIT DAH Listing 1 ;**********************************************************************
; Filename: picmrsrc.asm
; Date: Wednesday, December 30, 1998 10:28
; File Version:
; Author: Lawrence Foltzer
; Size: 365 bytes
;**********************************************************************
list p=16F84 ; list directive to define processor
#include <p16F84.inc> ; processor specific variable definitions
__CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _HS_OSC
; timing based on 1.12MHz RC oscillator configuration using 5.1Kohm + 100pf
; __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _RC_OSC
; timing based on 1.12MHz RC oscillator configuration using 5.1Kohm + 100pf
;**********************************************************************
;***** application specific equates
E equ .4
R_W equ .5
RS equ .6
code_in equ .7 ; PORTB,7 (also ICSPDATA and tone LED)
busy equ .3
BusyChk equ B'00100000'
FuncSet41 equ 0x22 ; get its attention
FuncSet42 equ 0x20 ; get it in right mode
DisplayOn equ 0x0c
EntryMode equ 0x06
InitDdRam equ 0x27
Ddra4Input equ B'00001111'
led equ .4 ; RA4 runs LED
Ddrb4Input equ B'10001111'
Ddrb4Output equ B'10000000'
ReadCntrl equ B'10100000'
pad1 equ 0x68 ; = 104
noise equ 0x02 ; 16 milliseconds or less is noise
Ithres equ 0x80
tabsize equ .4
; optest equ .16
;**********************************************************************
;***** ram allocation
ORG 0x0c
temp_w res 1
temp_status res 1
flags res 1
; "flags" bit assignment follows
DitDah equ 7 ; set (1) for DIT, clear (0) for DAH
overflow equ 6 ; slow code can cause counter overflow
PortaImage res 1
PortbImage res 1
timecnt res 1 ; this is increment in the ISR
period res 1 ; and transferred here on edge detection
thres res 1 ; computed symbol type decision threshold
codeword res 1 ; 1/0 representation of mickey morse
ditptr res 1 ; pointer to ditvals buffer
ditsum res 1
ditave res 1
ditvals res tabsize
dahptr res 1 ; pointer th dahvals buffer
dahsum res 1
dahave res 1
dahvals res tabsize
stack res 1 ; available RAM
;**********************************************************************
org 0x000
boot goto init
; ********************************************************************************
; this IRQ is entered every 4 milliseconds
org 0x004
ISR movwf temp_w
movf STATUS,W
movwf temp_status
incf timecnt,1
btfss STATUS,Z
goto isrjmp
bsf flags,overflow
decf timecnt,1
isrjmp movlw 0x74 ; 116
movwf TMR0
bcf INTCON,T0IF
exit1 movf temp_status,W
movwf STATUS
swapf temp_w,1
swapf temp_w,W
retfie
; ********************************************************************************
; morse tables follow ISR to keep them in 1st page, hopefully.
ditab clrf PCLATH
movf codeword,W
andlw B'00111111'
addwf PCL,1
retlw " "
retlw "e"
retlw "a"
retlw "i"
retlw "w"
retlw "r"
retlw "u"
retlw "s"
retlw "j"
retlw "p"
retlw 0x5f
retlw "l"
retlw 0x5f
retlw "f"
retlw "v"
retlw "h"
retlw "1"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x7c ; is "|" rather than "\" end of message
retlw 0x5f
retlw 0x5f ; _ = wait
retlw "2"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw "3"
retlw "!" ; acknowledge
retlw "4"
retlw "5"
retlw 0x5f
retlw "'"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw "."
retlw 0x5f
retlw 0x5f
retlw 0x22 ; quotation mark
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw "_"
retlw "?"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw "<" ; end of work
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
dahtab clrf PCLATH
movf codeword,W
andlw B'00111111'
addwf PCL,1
retlw " "
retlw "t"
retlw "n"
retlw "m"
retlw "d"
retlw "k"
retlw "g"
retlw "o"
retlw "b"
retlw "x"
retlw "c"
retlw "y"
retlw "z"
retlw "q"
retlw 0x5f
retlw 0x5f
retlw "6"
retlw "=" ; double dash
retlw "/" ; fraction bar
retlw 0x5f
retlw 0x5f
retlw ">" ; starting signal
retlw "("
retlw 0x5f
retlw "7"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw "8"
retlw 0x5f
retlw "9"
retlw "0"
retlw 0x5f
retlw "-"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw ";"
retlw 0x5f
retlw 0x5f
retlw ")"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw ","
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw ":"
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
retlw 0x5f
; ********************************************************************************
; next is for xtal clock
init movlw 0x04 ; divide XTAL OSC by 4 and then 32 for 4ms ticks
option ; later, load TMR0 with 256-140 and let overflow
; next is for rc clock
;init movlw 0x02 ; divide RC OSC by 4 and then 8 for ~4ms ticks
; option ; later, load TMR0 with 256-140 and let overflow
movlw Ddra4Input
tris PORTA
bcf PORTA,led ; on initially
movlw Ddrb4Input
tris PORTB
clrf PortbImage
clrf PortaImage
dlp1 decfsz PortaImage,1
goto dlp1
decfsz PortbImage,1
goto dlp1
call ChkBusy
movlw FuncSet41
movwf PORTB
call SendCmmd
movlw FuncSet42
movwf PORTB
call SendCmmd
movlw DisplayOn
call SendCmmd
movlw EntryMode
call SendCmmd
movlw InitDdRam
call SendCmmd
movlw .13
movwf ditptr
movlw ditptr
movwf FSR
ilp1 incf FSR,1
clrf INDF
decfsz ditptr,1
goto ilp1
movlw 0x18
movwf thres
clrf TMR0
clrf flags
clrf INTCON
bsf INTCON,T0IE
bsf INTCON,GIE
; ********************************************************************************
clrf PortbImage
clrf PortaImage
dlp1 decfsz PortaImage,1
goto dlp1
decfsz PortbImage,1
goto dlp1
call ChkBusy
movlw FuncSet41
movwf PORTB
call SendCmmd
movlw FuncSet42
movwf PORTB
call SendCmmd
movlw DisplayOn
call SendCmmd
movlw EntryMode
call SendCmmd
start btfss PORTB,code_in
goto start ; receiving a tone, exit on a space
bsf PORTA,led
; we are now receiving a space
start2 btfsc PORTB,code_in
goto start2 ; now we wait for a tone to time
; detected the start of a dit or dah
bcf PORTA,led ; light LED
clrf timecnt ; reset timer count
db1 movlw noise ; debounce (db) tone edge
subwf timecnt,W ; timecnt - noise
btfss STATUS,C
goto db1
w84spc1 btfss PORTB,code_in
goto w84spc1 ; tone still present
; tone stopped, but this is first so we don't know what it is. we assume it is a dah!
movf timecnt,W
&nbs movwf period
clrf timecnt
bsf PORTA,led
movlw 0x01
movwf codeword ; prep for 1st symbol
movf thres,W
subwf period,W ; period - thres
btfsc STATUS,C
goto isadah
isadit bsf flags,DitDah
call avedit
goto mainlp
isadah bcf flags,DitDah
call avedah
; now we wait for a tone to start so we can see what kind of space just passed
mainlp btfsc PORTB,code_in
goto mainlp ; space while set
movf timecnt,W
movwf period
clrf timecnt
; we are timing the current tone period now. so light the LED.
bcf PORTA,led ; let there be light
; ********************************************************************************
; so how long was the space? symbol or char
movf thres,W
subwf period,W ; period - thres
btfss STATUS,C
goto symbolspc
goto longspc
longspc rlf thres,W
subwf period,W ; period - thres
btfss STATUS,C
goto charspc
goto wordspc
; ********************************************************************************
symbolspc movlw noise
db3 subwf timecnt,W ; timecnt - noise
btfss STATUS,C
goto db3
db4 btfss PORTB,code_in
goto db4 ; wait for the tone to end
movf timecnt,W
movwf period
clrf timecnt
bsf PORTA,led
movf thres,W
subwf period,W ; period - thres
btfsc STATUS,C
goto itsadah
goto itsadit
; ********************************************************************************
itsadit call avedit
btfsc flags,DitDah
bsf STATUS,C
btfss flags,DitDah
bcf STATUS,C
rlf codeword,1
goto mainlp
; ********************************************************************************
itsadah call avedah
btfss flags,DitDah
bsf STATUS,C
btfsc flags,DitDah
bcf STATUS,C
rlf codeword,1
goto mainlp
; ********************************************************************************
charspc btfsc flags,DitDah
goto ditstart1
; goto dahstart1
; ********************************************************************************
dahstart1 call dahtab
goto printit
; ********************************************************************************
ditstart1 call ditab
printit call SendText ; printit, send to LCD
goto db1
; ********************************************************************************
wordspc btfsc flags,DitDah
goto ditstart2
; goto dahstart2
dahstart2 call dahtab
doit call SendText
movlw 0x20 ; space
call SendText
goto db1
ditstart2 call ditab
goto doit
; ********************************************************************************
avedit movf ditptr,W
addlw ditvals
movwf FSR
movf INDF,W
subwf ditsum,1
movf period,W
movwf INDF
addwf ditsum,1
rrf ditsum,W
movwf ditave
rrf ditave,1
movlw 0x3f
andwf ditave,1
incf ditptr,1
movlw 0x03
andwf ditptr,1
goto makethres
; ********************************************************************************
avedah movf dahptr,W
addlw dahvals
movwf FSR
movf INDF,W
subwf dahsum,1
movf period,W
movwf INDF
addwf dahsum,1
rrf dahsum,w
movwf dahave
rrf dahave,1
movlw 0x3f
andwf dahave,1
incf dahptr,1
movlw 0x03
andwf dahptr,1
makethres movf ditave,W
subwf dahave,W
movwf thres
bcf STATUS,C
rrf thres,1
movf ditave,W
addwf thres,1
return
; ********************************************************************************
SendText clrf PORTB
bsf PORTB,RS
goto Send1
SendCmmd clrf PORTB
Send1 movwf PortbImage
swapf PortbImage,w
andlw 0x0f
iorwf PORTB,1
bsf PORTB,E
bcf PORTB,E
movlw 0xf0
andwf PORTB,1
movf PortbImage,w
andlw 0x0f
iorwf PORTB,1
bsf PORTB,E
bcf PORTB,E
; ********************************************************************************
ChkBusy movlw Ddrb4Input
tris PORTB
SampleAgain movlw ReadCntrl
movwf PORTB
bsf PORTB,E
movf PORTB,w
movwf PortbImage
bcf PORTB,E
bsf PORTB,E
bcf PORTB,E
btfsc PortbImage,busy
goto SampleAgain
movlw Ddrb4Output
tris PORTB
return
;**********************************************************************
; this version is based on use of an NE567 tone decoder
; as input filter. decoder output is low when a tone is detected.
; advantages: immunity to adjacent channel signal, and amplitude variation.
; timing considerations, how long is a symbol?
; the reference symbol duration is the DIT!.
; a DAH symbol is 3 DITs long.
; the space between a pair of symbols is 1 DIT long.
; characters are groups of symbols and the spaces between them.
; the space between characters is 3 DITs long.
; the space between words is 5 DITs long.
; there are 50 symbol periods in the reference string: PARIS
; ". _ _ . . _ . _ . . . . . . "
; so at 12 WPM, we have 600 symbols in 60 seconds, ===> 0.1 sec / symbol
; @ 36 WPM, the shortest (DIT) symbol period is 33ms long.
; ********************************************************************************
; ********************************************************************************
; *****************************************
end
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