TinyFrame

TinyFrame is a simple library for building and parsing frames to be sent over a serial interface (e.g. UART, telnet etc.). The code is written to build with --std=gnu89 and later.

TinyFrame is suitable for a wide range of applications, including inter-microcontroller communication, as a protocol for FTDI-based PC applications or for messaging through UDP packets. If you find a good use for it, please let me know so I can add it here!

Frames can be protected by a checksum (~XOR, CRC16 or CRC32) and contain a unique ID field which can be used for chaining related messages. The highest bit of the generated IDs is different in each peer to avoid collisions. Peers are functionally equivalent and can send messages to each other (the names "master" and "slave" are used only for convenience and have special meaning in the demos).

The library lets you register listeners (callback functions) to wait for (1) any frame, (2) a particular frame Type, or (3) a specific message ID. This high-level API lets you easily implement various async communication patterns.

TinyFrame is re-entrant and supports creating multiple instances with the limitation that their structure (field sizes and checksum type) must be the same. There is a support for adding multi-threaded access to a shared instance using a mutex (via a callback stub).

Frame structure

All fields in the message frame have a configurable size (see the top of the header file). By just changing a definition in the header, such as TF_LEN_BYTES (1, 2 or 4), the library seamlessly switches between uint8_t, uint16_t and uint32_t. Choose field lengths that best suit your application needs.

For example, you don't need 4 bytes (uint32_t) for the length field if your payloads are 20 bytes long, using a 1-byte field (uint8_t) will save 3 bytes. This may be significant if you high throughput.

,-----+----+-----+------+------------+- - - -+------------,                
| SOF | ID | LEN | TYPE | HEAD_CKSUM | DATA  | PLD_CKSUM  |                
| 1   | ?  | ?   | ?    | ?          | ...   | ?          | <- size (bytes)
'-----+----+-----+------+------------+- - - -+------------'                

SOF ......... start of frame, 0x01
ID  ......... the frame ID (MSb is the peer bit)
LEN ......... nr of data bytes in the frame
TYPE ........ message type (used to run Type Listeners, pick any values you like)
HEAD_CKSUM .. header checksum
DATA ........ LEN bytes of data
DATA_CKSUM .. checksum, implemented as XOR of all preceding bytes in the message

Usage Hints

• All TinyFrame functions, typedefs and macros start with the TF_ prefix.
• Both peers must include the library with the same parameters (configured at the top of the header file)
• Start by calling TF_Init() with TF_MASTER or TF_SLAVE as the argument. This creates a handle. Use TF_InitStatic() to avoid the use of malloc(). If multiple instances are used, you can tag them using the tf.userdata / tf.usertag field.
• Implement TF_WriteImpl() - declared at the bottom of the header file as extern. This function is used by TF_Send() and others to write bytes to your UART (or other physical layer). A frame can be sent in it's entirety, or in multiple parts, depending on its size.
• If you wish to use timeouts, periodically call TF_Tick(). The calling period determines the length of 1 tick. This is used to time-out the parser in case it gets stuck in a bad state (such as receiving a partial frame) and can also time-out ID listeners.
• Bind Type or Generic listeners using TF_AddTypeListener() or TF_AddGenericListener().
• Send a message using TF_Send(), TF_Query(), TF_SendSimple(), TF_QuerySimple(). Query functions take a listener callback (function pointer)that will be added as an ID listener and wait for a response.
• To reply to a message (when your listener gets called), use TF_Respond() with the msg boject you received, replacing the data pointer (and len) with response.
• Manually reset the parser using TF_ResetParser()

Message listeners

Listeners are callback functions that are called by TinyFrame when a message which they can handle is received.

There are 3 listener types:

• ID listeners
• Type listeners
• Generic listeners

Listeners return an enum constant based on what should be done next - remove the listener, keep it, renew it's timeout, or let some other listener handle the message.

Examples

You'll find various examples in the demo/ folder. Each example has it's own Makefile, read it to see what options are available.

The demos are written for Linux, some using sockets and clone() for background processing. They try to simulate real TinyFrame behavior in an embedded system with asynchronous Rx and Tx. If you can't run the demos, the source files are still good as examples.