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i2c pullup resistors

 
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valemike
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i2c pullup resistors
PostPosted: Tue May 17, 2005 6:23 am     Reply with quote
i'm running at 33000 bps on the i2c bus; one pic acts as the master; another PIC as a slave; no other devices on the bus.

The slave's i2c code is isr driven, while the master's i2c code directly uses the MSSP libraries provided by CCS.

Once in a rare occasion, i don't get an ACK back from the slave following a master write of the slave address.

I suspect that my SDA and SCL edges are uneven in my circuit. Is it okay to lower the SDA pullup resistor value so that it swings to its logic level slightly ahead of SCL? I currently have 4.7K on both.
rnielsen



Joined: 23 Sep 2003
Posts: 852
Location: Utah

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PostPosted: Tue May 17, 2005 8:23 am     Reply with quote
Your parts won't be damaged if the pull-ups are lowered. I've had them as low as 1K before. Try experimenting with different values. If you're having problems with the integrity of your signals you might want to try a bus conditioner like the LTC1694-1 made by Linear Technology.

Ronald
valemike
Guest
PostPosted: Tue May 17, 2005 8:37 am     Reply with quote
Thanks Ronald. Since you responded to this post, I see now that it can be a problem.

Thanks,
Mike
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