c - How to design a function which identifies when "+IPD," is arrived from UART? -

i'm working tiva launchpad ek-tm4c123gxl , esp8266 wifi module.

when module gets wifi packet, sends microcontroller through uart port.

the format used esp8266 send packet (to uc via uart) is:

+ipd,n:xxxxx\r\nok\r\n

where:

• n length (in bytes) of data packet
• : indicates next byte first data byte
• xxxxx data packet
• \r\nok\r\n 6 bytes , useless me.

for example:

+ipd,5:hello\r\nok\r\n

here situation:

i'm working on existing project, can't change these 2 things:

1- uart module configured generate interrupt when receive fifo (of 16 bytes) half-full.

2- isr (interrupt service routine) handles interrupt:

• reads 1 byte uartdr (uart data register)

• saves variable

• calls function (called rx_data()) handle byte.

now, have write function, called rx_data(), in c language.

so, message coming form esp8266 module passed function, rx_data(), 1 byte @ time, , function must able to:

• identify header +ipd,

• read length n of data packet

• save data packet xxxxx (which located after : character , before first \r character) buffer

• discard final bytes \r\nok\r\n (these 6 bytes useless me, but, anyway, must read them remove them receive fifo)

i think work step step, i' m reasoning on:

how identify +ipd, , considering only 1 byte @ time passed function?

it's time make state machine. every time rx_data called, update state of state machine, , @ point know have received string "+ipd,".

the simplest thing work this, assuming byte received uart passed argument rx_data:

void rx_data(uint8_t byte) {     static uint8_t state = 0;     if (byte == '+') { state = 1; }     else if (state == 1 && byte == 'i') { state = 2; }     else if (state == 2 && byte == 'p') { state = 3; }     else if (state == 3 && byte == 'd') { state = 4; }     else if (state == 4 && byte == ',') {       state = 0;       handleipdmessage();  // received "+ipd,"     }     else { state = 0; } } 

you can see handleipdmessage() called if , if last characters received "+ipd,".

however, should consider writing more general state machine operate on lines instead of looking 1 string. easier write , more robust. when complete line received, call function named handlelinereceived() handle line. function have access buffer entire line, , parse in whatever way wants to. (just careful never write beyond end of buffer.)

by way, wouldn't putting logic in isr. it's best keep isrs simple , fast. if not doing already, store byte circular buffer in isr , read circular buffer in main loop, , every time read byte circular buffer call function rx_data function described above process byte.