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MSP4302274 I2C problems, UCBBUSY is high when battery powered

Micah Kurtz56
Intellectual 710 points

Hello,

I am using the MSP4302274 on a custom pcb as the I2C bus master, with three other slave devices. I am having some strange issues with the I2C bus.

Everything works fine when I upload the code and let it run freely, all the slaves are able to be found on the bus and send data to the MSP. However, the strangeness occurs when I apply power through an external battery, only once in a while everything works. Usually what happens is the UCBBUSY bit is set and the msp gets trapped. The only remedy I've found to fix this is to actually touch the reset line with my scope probe (or sometimes the SDA or SCL lines work too). 

When the UCBBUSY is busy both SDA and SCL are high. I am using 4.7k pull up resistors. I've tried removing all the slaves and communicating with them one at a time, and still the same issues happens. This type of behavior is only happening when I configure the I2C bus, if I am simply blinking LEDs then there is no issue at all. 

Why could be making the bus busy while a reset immediately fixes it?

Thanks!

  • 0
    Intellectual 710 points
    So just to add a bit more, I found that after testing my two boards have a stable VCC after about 150 uS. THe ez430-rf2500 which is what I am sort of basing my design from stabilizes in about 6 uS. I am wondering if the issue is my boards are taking too long to stabilize, which would explain why when the battery is initially plugged in it doesn't work, but a reset right after and the boards works. Can anyone confirm what the MSP would do in this scenario where VCC is taking this long to stabilize?
  • 0 in reply to Micah Kurtz56
    Intellectual 455 points

    Hi,

    It may be helpful if you can post oscilloscope shots of the SDA and SCL lines with maybe one slave connected. The bus activity might give us an indication as to what is going wrong.

    Regards

    Nishant Bhaskar

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