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I'm trying to interface a board level USB camera with a STM32 family microcontroller and send the image file to a central computer using CANbus. Just want to know if this is possible/ has been done before and how involved a task it would be.
I worked at a company where we sent live (low-resolution infra-red) video streams over CAN, but towards the end of my time there they shifted towards ethernet.
So it is possible, but certainly not what it is best suited for. The main advantages of CAN are that it is a multi-point, multi-master bus with built in arbitration. It is meant for short packets, typically 8 bytes (CAN FD allows you to increase that).
If your camera is USB, why not just get a USB repeater cable or USB-over-ethernet gateway?
If there is already a CAN network in place that you are piggy-backing onto then you need to consider what impact you will have on the existing traffic.
If you are starting from scratch then of course CAN will work but it would be an odd choice.
Depending on if its CAN or CANFD (Affects the maximum bulk transfer packet size) you have higher level protocol options to packetise your images and send them over canbus like any other block of data.
For just reguarlar CAN your after part of the standard called J1939.21 Data Link Layer, there are public versions of this floating around online, however due to the agreement when purchasing the standard, I am not able to share the specifics from what I have.
Its on pages 27-28 of the 2001 revision.
I'm totally a newbie in embedded software. Currently, I'm working on a project that implements an image processing pipeline on an ARM Cortex-M4 based MCU(board model: STM32F446RE).
I would like to be able to configure the parameters of the pipeline on the fly without actually update the entire firmware since we're using LoRa which has low bandwidth.
I have googled for several hours and could not find any valid solution. So could you please point me in a direction? Thank you very much.
BTW, I don't know if this is relevant, but I'm using FreeRTOS kernel with CMSIS RTOS API v2.
If you are asking this question, I would hope that either:
The board is still under design or
You have a board that was designed by someone who has thought about these issues.
If #2, speak to whoever designed the board, and find out what resources were put in, to handle these issues.
If #1, presumably you have input into the design.
Necessary resources:
Non-volatile storage: flash, eeprom, etc.
One or more ways to write parameters to that non-volatile storage
Desirable resource: communication line for input/output while running (serial is often used).
Once you have these resources, you do the following:
Design the variables, data structures, etc. to hold the parameters
Design your non-volatile storage, taking into account:
a. The features/limitations of your media (for example, flash memory generally requires an erase before writing. Erase takes time and must be done by sector, not individual bytes.
b. Verification: your program should have a way to verify that the non-volatile storage has valid values, not garbage, not all 0xFFs, and either fail or use defaults or some such, if it is not valid
Then you can write a program using this.
You need to consider how you will write the values to the non-volatile memory
during development
in production
They are not likely to be the same.
During development, you want to be able to easily change values. You may have a way to burn your flash chip via a JTAG. You may have a communications port which either runs some kind of simple CLI, accepts commands via some protocol, asks questions and reads the answers via a terminal emulator, etc. The program can then write the values to the non-volatile memory.
In production, you will likely want to burn the 'correct' values once, when setting up the system, without too much operator involvement.
This is just a starting guideline...as mentioned in the comments, your question is very general.
I want to interface two microcontrollers with a UART interface and I search a protocol to exchange data between them.
In practice, I want to exchange data periodically (ie: sensors reading) and also data on event (GPIO state). I have around 100-200 bytes to exchange every 100 milli second.
Does anybody know a protocol or library to achieve this kind of task ?
For now, I see protobuf and nano protobuff ? Is there something else ?
It would be nice if I could add a software layer over the UART and use "virtual data stream" like if it was a TCP/IP connection to N ports.
Any idea ?
Thanks
I think the most straight forward way is to roll your own.
You'll find RS232 drivers in the manufacturers chip support library.
RS232 is a stream oriented transport, that means you will need to encode your messages into some frameing structure when you send them and detect frame boundaries on the receiver side. A clever and easy to use mechanism to do this is "Consistent Overhead Byte Stuffing".
https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing
This simple algorithm turns zeros in your messages into some other value, so the zero-byte can be used to detect start and end of frame. If a byte gets corrupted on the way you can even resynchronize to the stream and keep going.
The code on Wikipedia should be easy enough even for the smallest micro-processors.
Afterwards you can define your message format. You can probably keep it very simple and directly send your data-structures as is.
Suggestion for a simple message format:
Byte-ID Meaning
---------------------------------
0 Destination port number
1 message type (define your own)
2 to n message data
If you want to send variable length messages you can either send out a length byte or derive the length from the output of the Constant Overhead Byte Stuffing framing.
By the way, UART/RS232 is nice and easy to work with, but you may also want to take a look at SPI. The SPI interface is more suitable to exchange data between two micro-controllers. It is usually faster than RS232 and more robust because it has a dedicated clock-line.
How about this: eRPC https://community.nxp.com/docs/DOC-334083
The eRPC (Embedded Remote Procedure Call) is a Remote Procedure Call (RPC) system created by NXP. An RPC is a mechanism used to invoke a software routine on a remote system using a simple local function call. The remote system may be any CPU connected by an arbitrary communications channel: a server across a network, another CPU core in a multicore system, and so on. To the client, it is just like calling a function in a library built into the application. The only difference is any latency or unreliability introduced by the communications channel.
I have use it in a two processor embedded system, a cortext-A9 CPU with a Context-M4 MCU, which communicate each other with SPI/GPIO.
Erpc can run over UART, SPI, rpmsg and network(tcp). even when using serial or SPI as transport tunnel, it can do bidirectional
calls and with very minimal footprint.
Simple serial point-to-point communication protocol
http://www.zipplet.co.uk/index.php/content/openformats_mise
It depends if you need master/slave implementation, noise protection, point-point or multi-point (and in this case collision detection), etc
but, as our colleague said, I would go with the simplest solution that fits the problem, following the KISS principle http://en.wikipedia.org/wiki/KISS_principle
Just add some header information like ID and length, if necessary CRC checking, and be happy :)
Try Microcontroller Interconnect Network (MIN) 1.0:
https://github.com/min-protocol/min
It has framing using byte-stuffing to keep receiver sync, 16-bit Fletcher's algorithm for checksum, an identifier for use by the application and a variable payload of up to 15 bytes.
There's embedded C code there plus also a Python implementation to make it easier to talk to a PC.
As the first answer starts, the simplest result is to roll your own. Define your header (the "format" above) as needed, perhaps including status information so each processor knows that the other is working properly. I have had success with a protocol that includes
2 byte ascii prefix and suffix such as "[" and "]" so that a
protocol analyzer can show you message boundaries.
The number of bytes.
The command ID (parsed to indicate what command handler to use.
Command arguments (I used 3 32 bit words).
A CRC or checksum to verify transfer integrity
The parser then recognizes the [* as the start of the message, and dispatches the body to the command handler for the particular command ID with the associated arguments as long as the checksum matches.
I'm working on a communication protocol between embedded devices. The protocol will definitely need new commands and fields in the future. What do I need to do to make sure I'm not painting myself into a corner?
This is a wide open question. Here are some random thoughts about it:
Leave spares.
Use a very basic header with a "number of bytes to follow" field.
If there are enumerated message types, make sure the type field can accomodate
growth.
If you use bitflags, leave spares.
Possibly include a "raw data" message, which can be used to wrap any protocol future generations think up.
In summary, Leave spares.
If at all possible, allow a human at one end of a cable to figure out what is at the other end of the cable.
Ideally, a human could hook up a dumb terminal and hit the keyboard three times (Enter Question-mark Enter), then a long, detailed message would come back describing what kind of machine it is, what is its model number, the name and phone number and web site of the organization that built it, the "official" protocol version number, and the unofficial build time:
__DATE__ ": " __TIME__
Also send the same detailed message every time the machine boots up.
If at all possible, try to design your protocol so that a human being with a dumb terminal can talk to your device.
The HTTP is one such human-readable protocol, and I suspect this is one of the reasons for its popularity.
Human-readable implies, among other things:
Limit yourself to characters that a human can read and type.
Avoid special control characters. Take advantage of the power of plain text.
Always send CR+LF at the end of each packet (as mandated by many Internet protocols).
Accept characters at any rate, from maximum-speed file upload from a PC to a non-touch-typing human slowly pecking at a keyboard.
You might also want to glance over the list of common protocols for embedded systems.
Perhaps one already meets your requirements?
Is there any reason to use something more difficult to decode than the standard Netstring format?
The question is a little too general for a clear answer. There are many aspects an embedded system may need to communicate like;
How many peers will it need to communicate with?
How much data does it need to communicate?
How tightly synchronized do the systems need to be?
What is the physical media for the protocol and what are the bandwidth limitations, and error susceptibility considerations?
All of these requirements and resource limitations will certainly constrain the system and then you can start to figure out what the protocol will need. Once you know these issues you can then project how some the requirements may change/expand in the future. From there you can design the protocol to accommodate(or not) the worst case use cases.
I would use HDLC. I have had good luck with it in the past. I would for a point to point serial just use the Asynchronous framing and forget about all of the other control stuff as it would probably be overkill.
In addition to using HDLC for the framing of the packet. I format my packet like the following. This is how options are passed using 802.11
U8 cmd;
U8 len;
u8 payload[len];
The total size of each command packet is len +2
You then define commands like
#define TRIGGER_SENSOR 0x01
#define SENSOR_RESPONSE 0x02
The other advantage is that you can add new commands and if you design your parser correctly to ignore undefined commands then you will have some backwards compatibility.
So putting it all together the packet would look like the following.
// total packet length minus flags len+4
U8 sflag; //0x7e start of packet end of packet flag from HDLC
U8 cmd; //tells the other side what to do.
U8 len; // payload length
U8 payload[len]; // could be zero len
U16 crc;
U8 eflag; //end of frame flag
The system will then monitor the serial stream for the flag 0x7e and when it is there you check the length to see if it is pklen >= 4 and pklen=len+4 and that the crc is valid. Note do not rely on just crc for small packets you will get a lot of false positives also check length. If the length or crc does not match just reset the length and crc and start with decoding the new frame. If it is a match then copy the packet to a new buffer and pass it to your command processing function. Always reset length and crc when a flag is received.
For your command processing function grab the cmd and len and then use a switch to handle each type of command. I also require that a certain events send a response so the system behaves like a remote procedure call that is event driven.
So for example the sensor device can have a timer or respond to a command to take a reading. It then would format a packet and send it to the PC and the PC would respond that it received the packet. If not then the sensor device could resend on a timeout.
Also when you are doing a network transfer you should design it as a network stack like the OSI modle. The HDLC is the data link layer and the RPC and command handling is the Application Layer.
I am building a small device with its own CPU (AVR Mega8) that is supposed to connect to a PC. Assuming that the physical connection and passing of bytes has been accomplished, what would be the best protocol to use on top of those bytes? The computer needs to be able to set certain voltages on the device, and read back certain other voltages.
At the moment, I am thinking a completely host-driven synchronous protocol: computer send requests, the embedded CPU answers. Any other ideas?
Modbus might be what you are looking for. It was designed for exactly the type of problem you have. There is lots of code/tools out there and adherence to a standard could mean easy reuse later. It also support human readable ASCII so it is still easy to understand/test.
See FreeModBus for windows and embedded source.
There's a lot to be said for client-server architecture and synchronous protocols. Simplicity and robustness, to start. If speed isn't an issue, you might consider a compact, human-readable protocol to help with debugging. I'm thinking along the lines of modem AT commands: a "wakeup" sequence followed by a set/get command, followed by a terminator.
Host --> [V02?] // Request voltage #2
AVR --> [V02=2.34] // Reply with voltage #2
Host --> [V06=3.12] // Set voltage #6
AVR --> [V06=3.15] // Reply with voltage #6
Each side might time out if it doesn't see the closing bracket, and they'd re-synchronize on the next open bracket, which cannot appear within the message itself.
Depending on speed and reliability requirements, you might encode the commands into one or two bytes and add a checksum.
It's always a good idea to reply with the actual voltage, rather than simply echoing the command, as it saves a subsequent read operation.
Also helpful to define error messages, in case you need to debug.
My vote is for the human readable.
But if you go binary, try to put a header byte at the beginning to mark the beginning of a packet. I've always had bad luck with serial protocols getting out of sync. The header byte allows the embedded system to re-sync with the PC. Also, add a checksum at the end.
I've done stuff like this with a simple binary format
struct PacketHdr
{
char syncByte1;
char syncByte2;
char packetType;
char bytesToFollow; //-or- totalPacketSize
};
struct VoltageSet
{
struct PacketHdr;
int16 channelId;
int16 voltageLevel;
uint16 crc;
};
struct VoltageResponse
{
struct PacketHdr;
int16 data[N]; //Num channels are fixed
uint16 crc;
}
The sync bytes are less critical in a synchronous protocol than in an asynchronous one, but they still help, especially when the embedded system is first powering up, and you don't know if the first byte it gets is the middle of a message or not.
The type should be an enum that tells how to intepret the packet. Size could be inferred from type, but if you send it explicitly, then the reciever can handle unknown types without choking. You can use 'total packet size', or 'bytes to follow'; the latter can make the reciever code a little cleaner.
The CRC at the end adds more assurance that you have valid data. Sometimes I've seen the CRC in the header, which makes declaring structures easier, but putting it at the end lets you avoid an extra pass over the data when sending the message.
The sender and reciever should both have timeouts starting after the first byte of a packet is recieved, in case a byte is dropped. The PC side also needs a timeout to handle the case when the embedded system is not connected and there is no response at all.
If you are sure that both platforms use IEEE-754 floats (PC's do) and have the same endianness, then you can use floats as the data type. Otherwise it's safer to use integers, either raw A/D bits, or a preset scale (i.e. 1 bit = .001V gives a +/-32.267 V range)
Adam Liss makes a lot of great points. Simplicity and robustness should be the focus. Human readable ASCII transfers help a LOT while debugging. Great suggestions.
They may be overkill for your needs, but HDLC and/or PPP add in the concept of a data link layer, and all the benefits (and costs) that come with a data link layer. Link management, framing, checksums, sequence numbers, re-transmissions, etc... all help ensure robust communications, but add complexity, processing and code size, and may not be necessary for your particular application.
USB bus will answer all your requirements. It might be very simple usb device with only control pipe to send request to your device or you can add an interrupt pipe that will allow you to notify host about changes in your device.
There is a number of simple usb controllers that can be used, for example Cypress or Microchip.
Protocol on top of the transfer is really about your requirements. From your description it seems that simple synchronous protocol is definitely enough. What make you wander and look for additional approach? Share your doubts and we will try to help :).
If I wasn't expecting to need to do efficient binary transfers, I'd go for the terminal-style interface already suggested.
If I do want to do a binary packet format, I tend to use something loosely based on the PPP byte-asnc HDLC format, which is extremely simple and easy to send receive, basically:
Packets start and end with 0x7e
You escape a char by prefixing it with 0x7d and toggling bit 5 (i.e. xor with 0x20)
So 0x7e becomes 0x7d 0x5e
and 0x7d becomes 0x7d 0x5d
Every time you see an 0x7e then if you've got any data stored, you can process it.
I usually do host-driven synchronous stuff unless I have a very good reason to do otherwise. It's a technique which extends from simple point-point RS232 to multidrop RS422/485 without hassle - often a bonus.
As you may have already determined from all the responses not directly directing you to a protocol, that a roll your own approach to be your best choice.
So, this got me thinking and well, here are a few of my thoughts --
Given that this chip has 6 ADC channels, most likely you are using Rs-232 serial comm (a guess from your question), and of course the limited code space, defining a simple command structure will help, as Adam points out -- You may wish to keep the input processing to a minimum at the chip, so binary sounds attractive but the trade off is in ease of development AND servicing (you may have to trouble shoot a dead input 6 months from now) -- hyperterminal is a powerful debug tool -- so, that got me thinking of how to implement a simple command structure with good reliability.
A few general considerations --
keep commands the same size -- makes decoding easier.
Framing the commands and optional check sum, as Adam points out can be easily wrapped around your commands. (with small commands, a simple XOR/ADD checksum is quick and painless)
I would recommend a start up announcement to the host with the firmware version at reset - e.g., "HELLO; Firmware Version 1.00z" -- would tell the host that the target just started and what's running.
If you are primarily monitoring, you may wish to consider a "free run" mode where the target would simply cycle through the analog and digital readings -- of course, this doesn't have to be continuous, it can be spaced at 1, 5, 10 seconds, or just on command. Your micro is always listening so sending an updated value is an independent task.
Terminating each output line with a CR (or other character) makes synchronization at the host straight forward.
for example your micro could simply output the strings;
V0=3.20
V1=3.21
V2= ...
D1=0
D2=1
D3=...
and then start over --
Also, commands could be really simple --
? - Read all values -- there's not that many of them, so get them all.
X=12.34 - To set a value, the first byte is the port, then the voltage and I would recommend keeping the "=" and the "." as framing to ensure a valid packet if you forgo the checksum.
Another possibility, if your outputs are within a set range, you could prescale them. For example, if the output doesn't have to be exact, you could send something like
5=0
6=9
2=5
which would set port 5 off, port 6 to full on, and port 2 to half value -- With this approach, ascii and binary data are just about on the same footing in regards to computing/decoding resources at the micro. Or for more precision, make the output 2 bytes, e.g., 2=54 -- OR, add an xref table and the values don't even have to be linear where the data byte is an index into a look-up table ...
As I like to say; simple is usually better, unless it's not.
Hope this helps a bit.
Had another thought while re-reading; adding a "*" command could request the data wrapped with html tags and now your host app could simply redirect the output from your micro to a browser and wala, browser ready --
:)