Richard-Gemmell
commented
3 years ago Hi zirafa,
This message comes from the finish() method in the wire.h implementation. It means that an I2C transfer didn't complete before the timeout expired. The transfer can be either a read or write but it's probably an attempt to read the MPR121 in this case.
The timeout is set by the timeout_millis field in i2c_driver_wire.h. The default value is 200 milliseconds which is a very long time in I2C terms. I would expect a typical transfer to take less than 1 millisecond. I would normally say that changing the timeout won't help but you mention that the errors are less common at higher clock speeds. In that case it might be worth trying a few different values just to see if it makes the errors more or less common.
If you're going to do that, please make sure you get enough measurements to be sure that there's a real difference. It's very easy with intermittent faults to convince yourself that something makes a difference when it's just a quirk of random noise. I'd suggest a minimum of 20 data points for each condition.
I can think of 2 possible reasons for the problem. (There may well be more!)
- An interrupt service routine (ISR) is stalling the I2C transfer
- The transfer is being aborted because of an electrical problem
The ISR problem is easy to avoid. ISR's are callback functions that sit outside loop(). You don't call them directly. Instead, some library calls them when something happens. These functions need to be ultra fast. They should avoid calling Serial.print() or other IO. This sort of work should be done in loop() instead.
Electrical problems are always an issue with I2C because the protocol doesn't have any built in error correction or retry mechanism. When something goes wrong you either get an incorrect data value or the transaction fails. In the worst case scenario the I2C bus can lock up and every following transfer fails as well. It's difficult to eliminate electrical problems but you may be able to make them very rare. (I have a system which generates less than 1 error per million transfers.)
- make sure there aren't any loose wires
- keep wires as short as possible (2 or 3 inches is ideal)
- make sure the Teensy and the sensors have a common ground and Vdd
- add pull up resistors if you have a lot of capacitance in your I2C wire (e.g. because of very long wires)
Does your system continue to work correctly after you've seen the error or do you have to reset it? i.e. is this issue just a nuisance or a major problem for you?
cheers, Richard
zirafa
aaronsherwood
Hello, thanks for writing this library. It definitely solved many of the blocking/speed issues I was encountering! However after swapping out the Wire.h library for Teensy4 I2C now I'm bumping up against an intermittent time out error: "Timed out waiting for transfer to finish."
I am running a Teensy 4.1 with audio shield, several MPR121 capacitive touch sensors, and an OLED. The error seems to happen while reading from the MPR121 sensors. If I increase the clock speed, the error happens less frequently. If I disable the MPR121 sensors entirely, the error disappears. My first thought was maybe there is a conflict between Teensy4 I2C and the Adafruit MPR121 library, but I'm not entirely sure. I suspect it might also be a conflict with the Audio library.
Any help is appreciated!