Using uart data protocol

[Ben Omlor]

I am working on an altimeter program.
The altimeter module uses uart data protocol.
The goal is to use the module with one of our gear alert boxes to check gear position and give a reminder if the plane gets to a set distance above the ground and the gears not down.
Right now, I have the module ran to a test board with a LCD display with hopes of establishing communication with the module and getting raw hex readings on the display. Once I get that I can work on decoding the info.
I am using a PIC16F18345 as the micro.
The uart protocol specs for the module are.
Baud Rate: 115200b/s
Data Bit: 8
Parity Bit: N
Stop Bit: 1
voltage level: 3.3

I am getting hex numbers on the display, but they do not seem to change with the movement above the module.
Program I have started is attached. I searched the forum for ideas and this format seemed to get me the closest.

Thanks for any help or ideas!
Ben

[Anobium]

Can you post the USART protocol and the datasheet for the module.

[mmotte]

Ben,
You should be looking for a "sync" character and then receive the packet of 7 characters.

Or is it a break for a long time?

Mike

[Anobium]

@mmotte. My thoughts are the same.

[Ben Omlor]

Hi,

Data sheet for module is attached. Page 14 has the data protocol.
This is my first attempt at something using this type of data transfer.
I am assuming I am missing some kind of pulse "sync" to tell it to respond.
The goal would be for it to continuously update the data to determine real time height.

Thanks!
Ben

[mmotte]

Ben, the altimeter is sending out packets continuously.
The beginning of the packet is 0xFE , so this is the "sync" character in this system.
Your program , to get into sync, will use HSerReceive to read and check if it is the beginning of a packet.
If not then read again.

if it is the beginning, 0xFE is the character, then continue by reading and filling the variables and checksum. They will be in the right places.

     SData(2) = HSerReceive
      SData(3) = HSerReceive
      SData(4) = HSerReceive
      SData(5) = HSerReceive
      SData(6) = HSerReceive

ver
altH
altL
snr
chksum

do checksum and see if valid

check snr and see if it is not valid

display altitude

go back and to short sync loop and wait for next packet

---

Your variables are bytes , not strings.

dim version as string
dim altitude as string
dim altitude2 as string
dim snr as string
dim checksum as string

So this part is wrong. It should be bytes and your array is bytes already

Also you could name the altitude as a word variable

dim  altitude as word

So then you only have to print one variable to the LCD or comparison. It will be handled as a 16bit number
altitude_H is the higher 8 bits
altitude is the lower 8 bits

I don't want to overload you, so I will quit there.

Mike

[Ben Omlor]

Thanks Mike!

I think I'm getting the jist.
A sync loop could look something like this?

do
wait 10 ms
SData(1) = HSerReceive
if sdata(1) = 0xFE then
SData(2) = HSerReceive
SData(3) = HSerReceive
SData(4) = HSerReceive
SData(5) = HSerReceive
SData(6) = HSerReceive
gosub display
end if

loop

Thanks,
Ben

[Anobium]

I would implement a ring buffer then you wont loss any incoming bytes. See the HELP for the ring buffer code.

[mmotte]

Ben,
Your program looks right for capturing the packet directly.

The data sheet doesn't tell the conversion from 16 bit altitude to the the reading you will output on the display. The overall range is 0.5M to 50 meter. But somehow you will have to combine the 2 bytes to make one number.
Method 1

Dim altitude as word

altitude_H = SData(4)
altitude   = SData(3)

the other way is :
altitude = SData(4) * 256 + SData(3)

With altitude being a word, the LCD routine will recognize that and print it as a number

Mike

[Ben Omlor]

Hi Guys,
Got slammed over the holidays so just getting back into this.
I believe with the following program I am syncing and getting data, but the display reads 255 for all values other than the Packet (sync) of 254.
I know its supposedly sending hex values, do I have to do any conversions to have it display properly?
Thanks for any help or ideas!

Ben

[Anobium]

You still need to implement blocking else the serial will crash through.

I still recommend a buffer ring and then you know each character arriving and you can sync in the buffer ring. The help has details of the buffer ring.

[Ben Omlor]

Now we're getting somewhere!
added #define USART_BLOCKING
now it returns varying values, but it seems to be alternating between a good value and 255 every time it reads.
I looked over the ring buffer help and I gotta admit it has me a bit intimidated. If you're willing to help answer some probably very dumb questions I'd like to take a go at it.
I see that it DIM's a sync byte but I do not see where you tell it what the syncbyte is supposed to be. And then how do I pull out the bytes that I need to use?

Thanks,
Ben

[mmotte]

Ben,
I agree ,you need blocking.

'USART settings
    #define USART_BAUD_RATE 115200
    #define USART_BLOCKING

I don't agree to needing a buffer. In this case it is being sent all the time. one after another. You don't want all of them you want the most recent. So you ask for it and at most you have to wait for the next packet.

sub get_val   'gets one packet
        index = 0
Look:
        HSerReceive temp           'Look for  start of message in tight loop
    If temp <> 0xFE then  goto Look

          HSerReceive Version
          HSerReceive Altitude
          HSerReceive altitude2
          HSerReceive snr
          HSerReceive Checksum
  end sub

So the main loop you just call the things to do

do Forever

get_val
;do you want to check checksum?
GroundAlt = altitude2 *256 + Altitude
Display
wait long enough to see the display

loop

Good luck
Mike

[Ben Omlor]

Thanks Mike!
That's got it! I was still getting an alternating good number and 254 on the display.
I added a checksum to filter out the bad read and now she's working great.
Now for the math to display it in feet for us Mericans. :-)

Thanks again,

Ben

[Anobium]

Excellent.