PR33-17 SCL Idle State Low

Products
1

Good afternoon all,

Simple question here, is the PR33-17 idle state truly supposed to be SDA high and SCL low? It’s odd because I’m used to the arduino default of idle meaning high for both.

To see this, all I did was plug the PR33-17 into the computer USB port and measure SDA/SCL relative to ground with a multimeter. If this is the case is there any way to change this? I have my system watch for a low clock signal to identify error conditions and this behavior is causing some confusion.

Thanks,
Ethan

2

SDA SCL should be high given that they are pulled high.

@ryan1 do you know what would this happen?

3

Here’s the Alpha Station Config panel in case that’s helpful. As you can see, there’s one device connected to the bus at address 0x25. And my multimeter still reads SDA = 4.96V, SCL = 0.02V.

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4

Hey team, just want to follow up to make sure this didn’t fall through the cracks. Any ideas/solutions?

5

@ryan1 is back now and should be able to check this out

1 Like
6

Please remove all external devices and remove the USB cables. You can then use Ohms on your meter to read between the +5V and SCL/SDA. You should read 4.5K or greater on each SCL and SDA lines. If not, please let me know.
Thanks,
Ryan

7

All seems okay with that.

4.72kOhm for 5V to SDA
4.75kOhm for 5V to SCL

Could there be some configuration setting/issue that could cause the processor to hold SCL low during inactivity?

8

Just to confirm, the device at idle is holding the SCL line low at all times and this is happening with nothing connected to the I2C port?

9

Correct as far as I can tell. When I plug the usb in for power (with nothing else connected to the bus), the SDA goes high and SCL stays low, according to a multimeter.

Tomorrow I’ll have access to both an oscilloscope and a new, unused PR33-17 module and I’ll see whether that shows the same.

To be clear, when I send messages through the serial port with the correct API, the I2C writes and reads work just fine. The only issue is that I use the low SCL state as an error state signifier so all external bus devices reset themselves to ensure they aren’t the culprit holding down the bus.

10

I just plugged a new (just out of package) PR33-17 into a usb and see the same thing, SDA sits high, SCL sits low. Do you see differently on your module?

11

I have confirmed the clock line will idle low on power-up. Unfortunately, the CPU is full and the project is closed, so no firmware changes can be made to this cpu. However, it may be possible to use a I2C multiplexer and switch to a channel you do not intend to use when you are finished with communications, this should release the clock line when not in use.

12

Okay, thanks for your help. As I mentioned, the part has generally worked well for me and will simply require a bit of an adjustment to overcome this limitation.