i2c help need with pull up resistor problem
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From: rbme on 11 Aug 2008 22:10 I've changed the pull up resistor to the 8451 required, 2.2-4.4k, and I don't have any other master. My device is a slave, thus my problem must be a bus fault. What is a bus fault and how can I fix it? Please Help!
From: Tobin on 12 Aug 2008 13:40 rbme, What appears to be happening is the 8451 sees that one or both of the I2C lines are being held low. It thinks that something else has control. What you can do for starters is to check the voltage of the scl and sda lines. You can do this with the 8451 removed from the system. Take a voltmeter, or an oscilloscope and probe the scl or the sda line of the slave device. Both should be idle high (either 3.3v or 5v). If either line is low, then you have a problem with your circuit. If both lines are high, then we need to look further. Tobin
From: rbme on 12 Aug 2008 16:40 I checked the values and the SCL was 2.7V and the SDA was 0.55V with the 8451 attached. I am not sure what you mean by having the 8451 not connect, because what would supply power then? Attached is the schematic I am using for my circuit from the manufacture of my sensor (Honeywell). From +5V from the 8451 goes into a 3.3V Regulator then to the two resistors then to the VDD's of the HMC6343. Thanks! Circuit.docx: http://forums.ni.com/attachments/ni/170/348797/1/Circuit.docx
From: Tobin on 12 Aug 2008 18:10 rbme, I'm presently downloading a microsoft office compatibility pack so I can view your attachement. But, in the mean time, if you are using the 8451 to supply power to your device, then just disconnect the scl and sda lines from the 8451 and leave the power connections intact. The pull up resistors still need to be connected between power and scl and sda of your device. I just want to find out if your device is pulling the sda line low by itself. Sometimes, an I2C slave device my hold the line low if it does not power-up correctly. I've just completed my download. I'll install it and then view your document. I may make further comments then. Tobin
From: Tobin on 12 Aug 2008 19:10
rbme, I've looked at the schematic that you posted. I've reviewed the spec for the slave device. Drawing power through the 8451 should be fine since it is only about 5 mA. The only things that I can suggest are: make sure that the power-up rise time for the device is at least 0.05v/msec (per the spec for the slave device). If you have a filter capacitor on the supply line that is too large, it may slow the power-up rise time. This could cause the device to latchup and hold the data line low. The other thing is to make sure that both the slave device and the 8451 share the same ground reference. I expect that is already the case since you are using the 8451 to supply power. If you measure the slave device while the clock and data lines are disconnected from the 8451 and they are fine (high) and then connect them to the 8451 and the data line latches low, then we may need to suspect the 8451. Do you have any other I2C slave devices that you could connect to the 8451? Even if you don't run the VI, the clock and data lines of the 8451 should be idle high if the pull-up resistors are attached. Tobin |