I2C Communication between two Microcontrollers

Il 05/07/2022 15:28, Neelakantappa M ha scritto:

I don't know very well these MCUs, but I think ST ecosystem gives you all the examples that you need to start understanding I2C communication between two MCUs.

Of course, you need an example of master I2C and an example of slave I2C.

The key is (at least) one of the MCU's needs a I2C driver that responds as a slave device. Then the other one, the master, can send a I2C command and possibly get an answer.

and a lot more sceptical about using it between boards. It can be good enough in simple cases, but all too often I have seen systems that

It is a protocol that works well with a microcontroller as the master communicating with slow slave devices (simple ADC/DACs, small EEPROMs, etc.) on the same board. It works poorly when you need more data transfer or higher speeds, and can be quite inefficient on many microcontrollers when they are acting as slaves.

It is also less than ideal for off-board traffic. For short range, with closely coupled ground and little electrical noise, it can work fine - but it does not work well over longer distances, and multi-master (or even multi-slave, with microcontroller slaves) can be very awkward. I

and the effort required to make it stable, noise-free and reliable meant it was more costly and complex than alternatives such as CAN or RS-485 would have been.

So my advice here is to think about where you are going in the future.

desk. But will it be the best choice for the final product - or its future versions? Obviously only you can answer that, but if it is not, then it is better to think about it now than later.

I agree with David for all points, considering that I2C and UART need both only two pins (at least for single master and single slave) and that modern MCUs most probably can configure pins for I2C or UART.

Moreover I add I hate I2C even for simple slave devices, such as EEPROM or ADCs/DACs. I prefer much more a simple SPI, even if I have to waste a pin for every slave.

I found that I2C peripherals embedded in most of MCUs are very complex and most of the time they aren't reliable. After some testing, I usually decide to write a bit-banging I2C code.

I have to agree here. I2C is conceptually simple, bu it is an edge triggered protocol and is very sensitive to noise unless you use buffer devices. It is excellent for master devices, but writing the slave software is much more complex to cover all cases.

A client of ours carefully wrote hardware drivers for their CPUs and put them on test. They saw one failure every hour or so. They then reverted to the bit-bang drivers supplied by us with the compiler. No failures in two weeks.

I have looked at these problems every few years or so, and the problem for hardware drivers has always been fast noise pulses on the I2C lines. By fast I mean up to several 10s of nanoseconds at a volt or so.

On the other hand we once did a hospital autoclave for which all I/O was over I2C and it was rock solid ... but we used the recommended buffer chips everywhere.

Stephen

Sorry for my stupid question, what do you mean with buffer devices? When you have master and slave on the same board, you put a wire between SDA/SCL pins and a couple of pull-up resistors.

Same for me. I2C hardware peripherals don't add any pro against a bit-bang solution (I'm talking always for masters), at least if you write blocking code that waits for the end of I2C transaction.

Yes, I think one solution is to reset and reconfigure the peripheral (if possible) when the software detects some problems.

Again, what are buffer chips?

themselves are not the big problem, it is the weak pull-up that leaves the lines very susceptible to noise and interference. SPI has edges, as do most protocols with a clock signal, but there the master drives high and low, giving a far more "solid" line.

features that complicate it, especially when used together. That includes multi-master, clock stretching, 10-bit addressing, and newer faster speeds. Good luck trying to make a microcontroller slave that works with all of that!

There is also the possibility of bus hang and invalid states. This can hit you during development - if you stop your microcontroller and

transaction, you can leave the slaves stuck - they may need a reset or power-cycle to recover.

On 2022-07-06 pozz wrote in comp.arch.embedded:

Try a google search for "I2C buffer". ;-)

There are plenty.

I would not choose I2C for connection between 2 parts of a device, but we have an existing device that uses I2C over a 2 meter cable and that works quite well. There is an I2C multiplexer (PCA9548A) in the remote part to select one of 8 I2C devices. (all same, so same adresses, so cannot use adressing to select a device). This device uses this buffer on both sides of the cable:

When the distance is further than normal (see spec) or the environment is very electrically noisy, you need buffer chips to eliminate/reduce noise problems, e.g.

In the autoclave, there were a number of switched mains devices.

The other application was in a powered railway carriage.

Stephen

The original Iic from Philips was for consumer equipment and has worked well for that sort of application, but it's not robust enough for professional work imho. I would never use it unless an io device needed it, such early Teletext devices. As you say spi is a far better sorted design...

Chris

I basically agree but things are not that bad as long as one does not push things too far. For me the worst part has been dealing with in-built I2C controllers, used two and both worked but each took me *days* to defeat - unlike the first time I used I2C some decades ago, bitbanging it from a HC11, which took me only an hour or two. Never used an MCU as an I2C slave yet, may do so soon but who knows. The peripherals I have used - some eeprom, RTC, ADC and perhaps some I can't think of now have all behaved; of course one has to deal with hanged bus situations, I have not seen a part which needs repower to get fixed (must have been lucky I guess). I have managed to upset the bus, being open drain, routing it too close to a flyback convertor switch, the latter doing 100V excursions "pretty fast" (tens of ns for the 100V I think). Changing the pullups on the I2c from 2k to 1k fixed that (still not that much of "too close", some luck again :).

====================================================== Dimiter Popoff, TGI

My experience is that to handle a "stuck" I2C bus, sometimes you need to be able to turn "off" the I2C controller and manually "bit-bang" up to (generally) 8 I2C clock pulses, until the slave that is stuck lets go of the I2C Data line, then you can force a START-STOP code (by pulling the data line low then high with the clock high) to get the bus into a usable state.

If that doesn't work, then you have a non-conforming device.

I do remember one time working with a slave that if you addressed it too soon after power up, it would go into clock streach mode (pulling down the clock) but never leave that mode. For that case the only option was to power down that device, and then power it back up and wait long enough.

Isn't that why SMBus was invented?

That handles it, but only if ALL your devices support it. The "Stuck Device" might be a device that doesn't support SMB bus, and might not normally need to, because it is just a simple slave that never clock streaches, so shouldn't be able to have a problem on a working bus.

The issue can happen if the controller get aborted mid-read-transfer, so the slave is driving the data bus (low) so a master can't assert a Start or Stop to reset the devices.