Question

RX & TX serial communication with GPS - PIC 16F84 - Hyperterminal

Asked by: dbuela

Hardware = a PIC 16F84, I use a crystal oscillator 4 MHz, in a multi-purpose test board with a MAX 232 to convert from TTL to CMOS. Also, a GPS receiver TFAG10, and a computer with Hyperterminal.
Software = Write the Assembly program in MPLAB IDE, simulate with MPLAB SIM and flash the program into the PIC with ICPROG.

The GPS receiver, transmit the data at 4800 bps, 8 bits, N,1 stop bit. When I connect the TFAG10 to the serial port of the computer. I can see the data comming through Hyperterminal (No problems there)

Assuming that at 4800 bps the lenght of the bit is 208 useconds, wrote a program to test that... below is the code (may be messy but works just fine)

                                list p=16F84
            radix hex
            org 0
status equ 0x03
porta equ 0x05
portb equ 0x06
count8 equ 0x0d
indf equ 0x00
fsr equ 0x04
            bsf status,5            ;bank 1
            movlw 0x00
            movwf portb            ;port b all out
            movlw 0x11
            movwf porta            ;ra0 y ra4 in
            bcf status,5            ;bank 0
start            movlw 0x11            ;lower limit is 0x12 but put -1 in fsr
            movwf fsr
            movlw 0x41            ;caracter "A"
            movwf 0x12            ;put it into this memory address
            movlw 0x42            ;caracter "B"
            movwf 0x13            ;put it into this memory address
            movlw 0x43            ;caracter "C"
            movwf 0x14            ;put it into this memory address
            movlw 0x44            ;caracter "D"
            movwf 0x15            ;put it into this memory address
            movlw 0x45            ;caracter "E"
            movwf 0x16            ;put it into this memory address
            movlw 0x46            ;caracter "F"
            movwf 0x17            ;put it into this memory address
            movlw 0x47            ;caracter "G"
            movwf 0x18            ;put it into this memory address
            movlw 0x48            ;caracter "H"
            movwf 0x19            ;put it into this memory address
            movlw 0x49            ;caracter "I"
            movwf 0x1a            ;put it into this memory address
            movlw 0x4a            ;caracter "J"
            movwf 0x1b            ;put it into this memory address
            movlw 0x4b            ;caracter "K"
            movwf 0x1c            ;put it into this memory address
            movlw 0x4c            ;caracter "L"
            movwf 0x1d            ;put it into this memory address
            movlw 0x4d            ;caracter "M"
            movwf 0x1e            ;put it into this memory address
            movlw 0x4e            ;caracter "N"
            movwf 0x1f            ;put it into this memory address
            movlw 0x4f            ;caracter "O"
            movwf 0x20            ;put it into this memory address
            movlw 0x50            ;caracter "P"
            movwf 0x21            ;put it into this memory address
            movlw 0x51            ;caracter "Q"
            movwf 0x22            ;put it into this memory address
            movlw 0x52            ;caracter "R"
            movwf 0x23            ;put it into this memory address
            movlw 0x53            ;caracter "S"
            movwf 0x24            ;put it into this memory address
            movlw 0x54            ;caracter "T"
            movwf 0x25            ;put it into this memory address
            movlw 0x55            ;caracter "U"
            movwf 0x26            ;put it into this memory address
            movlw 0x56            ;caracter "V"
            movwf 0x27            ;put it into this memory address
            movlw 0x57            ;caracter "W"
            movwf 0x28            ;put it into this memory address
            movlw 0x58            ;caracter "X"
            movwf 0x29            ;put it into this memory address
            clrf count8                  ; put count8 to zero
newbyte            incf fsr,1                  ;fsr=fsr+1
            bcf porta,1                  ;start bit
            nop
            nop
            nop
            nop
            nop                  ;several nops to meet timing
newbit            call delay                  ;go to delay
            btfsc indf,0                            ;is bit 0 = 0 on variable memory address?
            goto one                  ;no, is 1
zero            bcf porta,1                  ;yes, send a 0 on RA1
            rrf indf,1                  ;move all bits down one place
            incf count8,1            ;count8=count8+1
            btfss count8,3            ;is it 8?
            goto newbit            ;no, go and find a new bit
            goto stopbit            ;yes, go and send stop bit
one             bsf porta,1                  ;send a 1 on RA1
            rrf indf,1                  ;move all bits down one place
            incf count8,1            ;count8=count8+1
            btfss count8,3            ;is it 8?
            goto newbit             ;no, go and find a new bit
stopbit                       call delay                                        ;delay
            bsf porta,1                  ;send a 1 on RA1
            call delay                  ;delays between bytes
            call delay                  ;
            clrf count8                  ;put count8 to zero
            movlw 0x29            ;the final character is on memory 0x29
            subwf fsr,0            ;w= fsr-0x29
            btfsc status,2            ;did fsr reach the final character ?
            goto start                  ;yes, all over again
            goto newbyte            ;no, go and get a new byte
delay            movlw 0x40
            movwf 0x0e
loop            decfsz 0x0e,1
            goto loop
            return
            end

So, if my GPS receiver and the program above both can be viewed by the Hyperterminal (4800, 8,N,1) then I assume that the lenght of the bit that the GPS receiver gives me is the same as my program (208 useconds = 1/4800).

Well, and here is my problem, I CAN NOT make it work assuming 208 useconds... instead it works when I put a delay between 40 and 100 useconds (anywhere between those values) using this program:
RA0 is connected to the GPS and RA1 is connected to the computer.

            list p=16F84
            radix hex
            org 0
status equ 0x03
porta equ 0x05
portb equ 0x06
            bsf status,5      ;bank 1
            movlw 0x00
            movwf portb      ;portb all out
            movlw 0x11
            movwf porta      ;ra0 and ra4 in (don't use ra4 at all)
            bcf status,5      ;bank 0
start            movlw 0x00
            movwf portb      ;port b all off (I don't use them)
search            btfsc porta,0      ;is there a 0 in RA0 ? (in)
            goto one            ;no, go to one
zero            bcf porta,1            ;yes, then put a 0 in RA1 (out)
            call delay            ;go delay
            goto search      ;back to search
one            bsf porta,1            ;put a 1 in RA1 (out)
            call delay            ;go delay
            goto search      ;back to search
delay            movlw 0x1e               ;maximum value for the transmission to be OK
            movwf 0x0e
loop            decfsz 0x0e,1
            goto loop
            return
            end

So, this program also works fine, but I can't understand why ....because the delay is way too short for the suposed 4800 bps from the GPS... and if in deed the bit is shorter (then it won't be 4800 !! as the specifications state)... why is the GPS working fine with Hyperterminal at 4800 ?

Feel free to treat me as ignorant, I don't care as long as answer is reached. :-)

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Asked On
2006-09-27 at 23:23:47ID22005694
Tags

pic

,

gps

,

serial

Topic

Assembly Programming Language

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: PaulCaswellPosted on 2006-09-28 at 10:58:46ID: 17621450

Hi dbuela,

Did you forget about the start bit and the stop bit? This generally makes it 10 bits per byte. Sorry, I havent looked at your code.

Also, most systems cant handle only one stop bit, usually 1 1/2 or even 2 is required but at 4800 this shouldnt be a problem.

Paul

 

by: mokulePosted on 2006-09-28 at 16:44:03ID: 17623885

Hi

1. You should include file to use some defined constants for that processor
      #include    <p16f84.inc>   ; processor specific variable definitions
2. What does that mean that this program works?
    What they should do?
    It's not quite clear for me.
3. I can't see any port direction settings.
   You don't set TRISA and TRISB.
   To tell the truth I don't understand how can You output anything from processor.
4. Is the second program meant to retransmit RS data?
  If so, there is no need for call delay at all.
  It works more or less this way.
  You read some bit, try to set output the same value.
  You call delay which is shorter then normal bit duration.
  So after the delay You read the same bit once more.
  So if You set delay for 104Us You read each bit twice, if You set 68 You read each bit 3 times and so on.
  This doesn't do any harm as long as You only want to retransmit.

Marek

 

by: dbuelaPosted on 2006-09-28 at 21:02:05ID: 17625084

Hi mokule, thanks for taking the time.

1. Don't understand why should I include the p16f84.inc file, most probably that's a good programmer practice, but honestly I don't think my problem is there (if you think so, let me know !)

TRISA and TRISB are the same memory address as PORTA and PORTB, just different bank... (bank 1 means TRISA/B)

The function of the second program is:
It looks at the input
Whether a 0 or a 1 it reflects it to the output
Then delays until it goes to look at the input again.

My logic is that I vary the delay as high as possible, as long as I can see no mistakes on the ouput of the microcontroller through Hyperterminal.

Once I started to see troubles on Hyperterminal means that I delay too much, (the normal output start to show funny characters).

Putting a 0x1e on the delay gives about 100 useconds between readings and the output is just fine (that is what I mean saying "it works"), when increasing the delay to 0x20 I started to see some funny things on Hyperterminal therefore I assume that I'm jumping input bits... the more I increase the delay the worse is the output.

I agree with you that "in theory" my delay of 0x1e is going to read the bit twice, but I kept increasing the delay up to 0x40 (or more, actually that's how I started) makes the output unreadable at all, mind you the time between readings was 208 useconds (according to the simulator trace)

The purpose of the second program is to determine the lenght of a bit (in theory 208 useconds)

Thanks again

 

by: mokulePosted on 2006-09-29 at 03:05:19ID: 17626174

Hi

You should include p16f84.inc to easier programming and make program more readable.
Of course it has nothing to do with program functioning.

TRISA issue.
You're right. I know it must be somewhere :) if the program function somehow.
But using TRISA and PORTA names is a must for program readability.

Now to the program functioning
The whole delay idea is wrong.
In practice You can't assume the bits received are equal in length. Your quartz is not also 100% stable.

So my advice.
If You want only to retransmit You can remove delay at all.
As a result You will check each bit many times.

But if You want to register incoming bits do it like this
1. wait for starting 0
2. delay 104Us, You should be now in half start bit
3 output 0
4 delay 208Us - You're now in half of first bit
5 test input, and set output appropriately. Save received bit.
6. Repeat 4 and 5 nine times. With the exception that the last bit is stop bit so don't save it.

i hope this helps


 

by: dbuelaPosted on 2006-09-29 at 03:15:40ID: 17626207

Thanks mokule,

My head was burning with this issue last week...
I figured it out today and tested and worked perfectly, but didn't have the time to post the answer.
You did that for me, you deserve the points (too many !! it wasn't so difficult after all !! :-)) thank you for taking the time to read my programs.

Most probably I will have more questions.

 

by: mokulePosted on 2006-09-29 at 03:24:43ID: 17626232

And here is the code

          list p=16F84
     #include    <p16f84.inc>   ; processor specific variable definitions

loopcnt equ 0x30

          radix hex
          org 0
          bsf status,5     ;bank 1
          movlw 0x00
          movwf TRISB     ;portb all out
          movlw 0x11
          movwf TRISA     ;ra0 and ra4 in (don't use ra4 at all)
          bcf status,5     ;bank 0
start          movlw 0x00
          movwf portb     ;port b all off (I don't use them)

startbitwait
          btfsc porta,0     ;is there a 0 in RA0 ? (in)
          goto startbitwait          ;no, wait for start
          movlw    .10     ; 8 bits + start + stop
          movwf    loopcnt
          call   delay104
nextbit
          btfsc porta,0     ;is there a 0 in RA0 ? (in)
          goto one          ;no, go to one
zero          bcf porta,1          ;yes, then put a 0 in RA1 (out)
          call delay208          ;go delay
          goto nextbit     ;back to search
one          bsf porta,1          ;put a 1 in RA1 (out)
          call delay208          ;go delay
          decfsz     loopcnt,F
          goto nextbit
          goto startbitwait

delay104
         ....         ; here should be some code :)
         return
delay208
         ....         ; here should be some code :)
         return
          end

 

by: mokulePosted on 2006-09-29 at 03:28:45ID: 17626242

oh,
 I didn't see Your post while writing

thanks

20120131-EE-VQP-002

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