GPS automatic baud rate detection

[pjalocha]

could be done by switching between several standard rates as long as no valid NMEA frames are detected. Once the NMEA frames start arriving the speed should stay.
Algorithm is easy and should achieve correct speed within 10-20 seconds.

It can be very convenient, as one needs not to check eveyr time if the GPS speed matched the configured tracker speed.

[pyrog]

Could use one of following algorithms:

STM32 boot loader baud rate detection

"Bootloader measures the signal length between rising edge of first 1 bit in 0x7F to the falling edge of the last 1 bit in 0x7F to deduce the baudrate value"

Source: STM32 microcontroller system memory boot mode

First character received by the STM32 boot loader:
0111 1111 ← 0x7F
_<------>

First NMEA sentence sended by the GPS
$GPGGA,184155.422,,,,,0,00,,,M,0.0,M,,0000*5E

0010 0100 ← 0x24 ← $
__<--->__

The pattern to measure could be
0010 0100 ← 0x24 ← $
0100 0111 ← 0x47 ← G
0101 0000 ← 0x50 ← P
0100 0111 ← 0x47 ← G
0100 0111 ← 0x47 ← G
0100 0001 ← 0x41 ← A

0010 0100 0100 0111 0101 0000 0100 0111 0100 0111 0100 0001

Detect a pattern sended at differents speed

// s is the first string received at 115200
 
switch (s) {
case 'OGN':
  baudrate = 115200;
  break;
case '`b#�':
  baudrate = 57600;
  break;
case '`sÌrÈ':
  baudrate = 38400;
  break;
…
}

• GPS send at 115200 bauds, tracker receive at 115200

OGN Tracker Console.
$PSRF150,1*3E
$GPGGA,193512.912,,,,,0,00,,,M,0.0,M,,0000*51
$GPGSA,A,1,,,,,,,,,,,,,,,*1E

• GPS send at 115200 bauds, tracker receive at 57600

b#� ¤��á§Çäb!ᧁ��§@ó$ �¦󆦥æ¥ÆÆÆÄ楄ÆFƆ�FÆƄ��@ó$ �ÆÆÆÆÆÆÆæb$ "�󆆦¥æ¥ÆÁÆÆÆ楅�¤ÆfƇEó$ �¦󆦥æ¥ÆÆÆÄ楄ÆFƆ�FÆƄ��Aó$ �ÆÆÆÆÆÆÆæb$ "�󆆦¥æ¥ÆÁ

• GPS send at 115200 bauds, tracker receive at 38400 bauds
  sÌrÈ=̤ÿàªpbડ¨�ábbqrppRpRbàqó¨ᨊurrrrrb¡¨j±X�ñ99±+8i(ý¨񱨊ábbprppRpRbàqó¨ᨊurrrrrb¡¨j±XGð99±+8i(ý�çÁ=ó𱈨�ábâqbavpñaRqprapRàq!rèᨊõâbbbbbrrâqrq⡨j????X�0°±\ruâbññqrÃ�çÁ{ó∨�ábòpbavpñ¡???
  `
• GPS send at 115200 bauds, tracker receive at 19200 bauds

zFÿþþXR                                                                      
         JBþHJ                                                               
               JPXþ                                                          
                     HH                                                      
                       þHJJJZþ°yHXBZJRJX                                     
                                          �BXJHXJRHX        
                                                     RüBXXBþ
                                                            HJRHX
                                                                  RüXJ
                                                                       B
                                                                      þHBRþXBR

If the GPS don't start with the same string, "simply" use a repeating pattern i.e. ($GPGGA,󆦥æ¥ÆÆÆ,ább)

GPS Autobaud algorithm on Arduino

https://github.com/rob42/FreeboardPLC_v1_2/blob/master/Gps.cpp#L89-L148