Debugging 96Boards I2C

I was originally just going to post this to one of the 96boards mailing list, but it got sufficiently interesting that I thought I’d make it a blog post instead. I’ve been working on making i2c on the 96Boards sensors adapter work properly and I’ve made some progress. The problem that user have run into is that the Grove RGB LCD module won’t work when connected to one of the baseboard’s I2C busses. I pulled out the oscilloscope today to investigate.

The LCD module is particularly useful for testing because it actually has 2 i2c devices embedded in it; an LCD controller at address 0x3e, and an RGB controller at 0x62. The two devices operate independently with different electrical properties.

​On Hikey+sensors (TXS0108 level shifter), the RGB device will work, but only after pulling the ribbon cable apart to reduce crosstalk due to insufficient pullups. However, the LCD causes the entire bus to lock up, and no further transactions will work.

On Hikey+pca9306 the LCD isn’t detected and the RGB works correctly (undetermined if there are crosstalk issues)

​The traces below show both sides of the level shifter. Green and blue on the top for the data line. Orange and purple on the bottom with the clock.​

First, what I saw on using Hikey+pca9306+RGB:

Screen Capture
RGB transaction via PCA9306

And with the LCD:

Screen Capture
LCD transaction via PCA9306

In both traces you can see the start condition (data goes low while clock is high), the 7 bits of address (7 rising clock edges), the R/W bit (1 rising clock), and then the acknowledgement bit driven by the device. If the controller doesn’t see the device drive the data line low on the 9th clock, then it decides the device isn’t there and it terminates the transaction. It is easy to recognize the ack bit because the device has a different drive strength and the voltage level is different.

The RGB controller is a happy little device and it jumps at the chance to drive the data line low. It goes down pretty close to 0V. The LCD on the other hand is sulky and doesn’t drive the line quite as low as the controller can. About to 1V. 1V is recognized fine as logic low on a 5V device, but with 1.8V it is not even less than half. The way the pca9306 level shifter works is there are pull-up resistors on either side of the device that draws each side up to its respective high level. In this case, 1.8V and 5V. When either side gets driven low, the level shifter begins to conduct and the other side also gets drawn down to the same voltage, but it can only go as low as the voltage it is driven to. If it only gets driven down to 1V, then it will never get low enough for a 1.8V controller to recognize it as a low state.

It may be that with weaker pull-ups the LCD will be able to drive to a lower voltage level. I’ll need to experiment more, but in the mean time let’s move onto the Sensors board. Back to the traces:

First, here is a transaction to address 0x63 with no device present:

Screen Capture
No device

​Looks perfectly normal so far. Next, the RGB device at address 0x62:

Screen Capture
RGB

Also behaving the same way as it did with the pca9306. Finally, an LCD transaction:

Screen Capture
LCD

Again we see the start condition, the 7 data bits and 1 r/w bit, but the ack bit looks weird. The LCD successfully drives the data bit low enough to be recognized, but then something weird happens. The data line stays low and the clock stops running. I don’t know actually know what is happening here, but I’ve got my suspicions. The LCD is continuing to drive the data line low, (you can tell by the slightly different voltage level) but keeping data low should not stop the clock. I suspect the txs0108 is getting confused and driving the clock line high. I’ve come across reports from others having trouble with the txs010x series on i2c. It has ‘one-shot’ accelerators to reduce rise time by driving the line high. I don’t know for sure though.

On the plus side, I now know that the Hikey I2C busses are working correctly. Now I need to decide what to do next. Aside from the i2c problem, Rev B of the sensors board ready for manufacturing. I either need to make the txs part work, or rework the design to use a pair of pca9306s. I think I’ll try weaker pull-ups on the pca9306 breakout board first and see how that goes. Sadly, I blew up the i2c drives on my Hikey board while experimenting today, so I need to do the same experiments with my Dragonboard 410c.

Dear lazyweb, do you have any other suggestions on things to try?

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