ardupilot/libraries/AP_Scripting/examples/NMEA-decode.lua

117 lines
3.2 KiB
Lua

-- Script decodes, checks and prints NMEA messages
-- luacheck: only 0
-- find the serial first (0) scripting serial port instance
local port = serial:find_serial(0)
if not port then
gcs:send_text(0, "No Scripting Serial Port")
return
end
-- begin the serial port
-- NMEA is usually 4800 or 9600
port:begin(4800)
port:set_flow_control(0)
-- table for strings used in decoding
local term = {}
local term_is_checksum = false
local term_number = 1
local checksum = 0
local string_complete = false
-- maximum number of terms we expect in the message
local max_terms = 15
-- maximum length of terms we expect
local max_term_length = 5
-- decode a basic NMEA string, only check the checksum
local function decode_NMEA(byte)
local char = string.char(byte)
if (char == ',' or char == '\r' or char == '\n' or char == '*') and not string_complete then
if char == ',' then
-- end of a term, but still counted for checksum
checksum = checksum ~ byte
end
-- null terminate and decode latest term
if term_is_checksum then
-- test the checksum
string_complete = true
return checksum == tonumber(term[term_number],16)
else
-- we could further decode the message data here
end
if char == '*' then
-- the next characters make up the checksum
term_is_checksum = true
end
-- nothing in current term, add a space, makes the print work
if not term[term_number] then
term[term_number] = ' '
end
-- move onto next term
term_number = term_number + 1
return false
end
if char == '$' then
-- sentence begin
-- clear all flags, reset the term table and checksum
term_is_checksum = false
term_number = 1
checksum = 0
term = {}
string_complete = false
return false
end
-- ordinary characters are added to term
-- if we have too many terms or they are too long then don't add to them
if term_number < max_terms then
if term[term_number] then
if string.len(term[term_number]) < max_term_length then
term[term_number] = term[term_number] .. char
end
else
term[term_number] = char
end
end
-- update the checksum
if not term_is_checksum then
-- checksum is bit wise xor of all characters in the string before the checksum
checksum = checksum ~ byte
end
return false
end
-- the main update function that is used to read in data from serial port
function update()
if not port then
gcs:send_text(0, "no Scripting Serial Port")
return update, 100
end
local n_bytes = port:available()
while n_bytes > 0 do
local byte = port:read()
if decode_NMEA(byte) then
-- we have got a full NMEA message that has passed the checksum
-- concatenate back to full message and print
-- don't print the checksum
gcs:send_text(0, table.concat(term,",",1,#term-1))
end
n_bytes = n_bytes - 1
end
return update, 100
end
return update, 100