AP_Scripting: add NMEA decode example

libraries/AP_Scripting/examples/NMEA-decode.lua

@@ -0,0 +1,106 @@
+-- Script decodes, checks and prints NMEA messages
+
+-- 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
+port:begin(9600)
+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
+
+-- 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 == '*' 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
+            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
+
+        -- 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 = {}
+        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