I2C Interrupt in MotorWare

Benjamin Hudson

Other Parts Discussed in Thread: MOTORWARE

Hi there,

I'm trying to use I2C to control two LAUNCHXL-F28069Ms. As far as I can see, communication works fine until the PWMs start at which point the I2C gets borked. The weird part is this communication only gets messed up when both motors are on and both boards are connected to the I2C bus. Has anyone successfully implemented I2C in a motorware project? Does anyone have any idea why this could be happening?

Thanks so much,
Ben

EDIT: is it possible that it could be an electrical problem? Enough noise on the I2C bus when there are two motors running but not one?

over 10 years ago

0 Adam Haun over 10 years ago

TI__Expert 8950 points

Ben,

What resistor value are you using for the SDA and SCL pull-ups? Have you tried scoping SDA and SCL to check for electrical interference from the motors?

You could also check how much time you're spending in PWM and ADC interrupts. They have higher priority than the I2C interrupts.

0 Benjamin Hudson over 10 years ago in reply to Adam Haun

Intellectual 555 points

Hi,

I'm using the internal pullups on the board. There is a lot of noise on the lines and I will try isolating them to see if it helps. I'm building off one of the labs and haven't added very much to the interrupts? How much faster are fixed-point operations than floating-point ones on the F28069M (with FPU)?

Ben

0 Adam Haun over 10 years ago in reply to Benjamin Hudson

TI__Expert 8950 points

In that case, the bus is probably only being driven by the internal pull-ups on the MCU. Those aren't strong enough to guarantee proper I2C performance. Try adding 1k pull-up resistors to SDA and SCL and see if that fixes the problem.

0 Benjamin Hudson over 10 years ago in reply to Adam Haun

Intellectual 555 points

Thanks Adam, that did the trick. Is there a rule of thumb I can use to calculate the pullup resistance needed? I plan on adding more boards to the same bus.

Ben

0 Adam Haun over 10 years ago in reply to Benjamin Hudson

TI__Expert 8950 points

Ben,

The necessary resistance depends mainly on the bus capacitance. We have an app note on this (SLVA689):

www.ti.com/.../getliterature.tsp

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I2C Interrupt in MotorWare