i2c: half amplitude on SDA

Question

I try to detect a rtc device on an i2c bus, with the i2cdetect utility from the i2ctools package. As I cannot see anything when scanning the i2c bus, I use an oscilloscope and on the 9th SCL's rising edge (ACK bit), I get a half amplitude on the SDA signal.

Further details:

What voltage are your measuring normally, and what in the unusual case?

normal voltage: 0V for a 0 logical, 3.3V for a 1 logical. In the unusual case I measure 1.4V (almost the half voltage of an 1 logical in the normal case)

Do you have a pullup resistor and open collector drivers the way you should?

Yes, the SDA and SCL lines are pulled up with a 4k7 resistor and the rtc device is configured as open drain (CMOS).

Do both the master and slave operate at the same voltage?

Yes, at 3.3V

@Martin Thompson: Thanks. So, here is the schematic of my i2c bus (actually, only a rtc device is connected on).

    3.3V                            3.3V                3.3V                3.3V
        ____                            ____                ____                ____
         |                                |                   |                   |
         |                              +--+                +--+                  |
         |                          4k7 |  |            4k7 |  |                  |
+-----------------+                     |  |                |  |            +--------------+
|                 |                     +--+                +--+            |              |
|  FPGA Cyclone 4 |                       |                   |             |   Real time  |
|     (Altera)    |                       |                   |             |   clock      |
|       GPIO      | ---SDA----------------+-------------------|-------------|   ST m41t83  |
|                 |                                           |             |              |
|                 | ---SCL------------------------------------+-------------|              |
|                 |                                                         |              |
|                 |                                                         |              |
+-----------------+                                                         +--------------+
        |                                                                           |
        |                                                                           |
      _____                                                                       ______
        0V                                                                          0V

and this is the screenshot of the oscilloscope, which picks up the SDA and SCL signals:

SDA
________        ______________         _________________________________                                        __       ___________
        |      |              |       |                                 |                                      |  |     |           |
        |      |              |       |                                 |                                      |  |     |           |
        |      |              |       |                                 |                                 _____|  |     |           |
        |      |              |       |                                 |                                |        |     |           |
        |______|              |_______|                                 |________________________________|        |_____|           |_______________


SCL 
____________     ___     ___     ___     ___                                 ___     ___     ___     ___     ___     _______________________________
            |   |   |   |   |   |   |   |   |                               |   |   |   |   |   |   |   |   |   |   |
            |   |   |   |   |   |   |   |   |                               |   |   |   |   |   |   |   |   |   |   |
            |   |   |   |   |   |   |   |   |                               |   |   |   |   |   |   |   |   |   |   |
            |   |   |   |   |   |   |   |   |                               |   |   |   |   |   |   |   |   |   |   |
            |   |   |   |   |   |   |   |   |                               |   |   |   |   |   |   |   |   |   |   |
            |___|   |___|   |___|   |___|   |_______________________________|   |___|   |___|   |___|   |___|   |___|

These signals are obtained when I send a request to my rtc device at address 0x68 with the i2cdetect utility (i2cdetect 0 0x68 0x68) from a linux shell.

By the way, excuse me for the poor ascii design, but as I am new on the forum I cannot post images. Hope it will be understandable ;-)

Clarification about the "screenshot": the SCL's and SDA's amplitude is about 3.5V and on the unusual case (on the 9th scl's rising edge) it is 1.4V

What voltage are your measuring normally, and what in the unusual case? Do you have a pullup resistor and open collector drivers the way you should? Do both the master and slave operate at the same voltage? — Chris Stratton
Hi Chris, thanks for your reply. I'll answer your questions: What voltage are your measuring normally, and what in the unusual case? => normal voltage: 0V for a 0 logical, 3.3V for a 1 logical. In the unusual case I measure 1.4V (almost the half voltage of an 1 logical in the normal case) Do you have a pullup resistor and open collector drivers the way you should? => Yes, the SDA and SCL lines are pulled up with a 4k7 resistor and the rtc device is configured as open drain (CMOS). Do both the master and slave operate at the same voltage? => Yes, at 3.3V — jrm
Can you post a schematic of your entire I2C bus, showing all the resistors, power supplies, master and slaves? Also, post a screenshot from the oscilloscope if possible, describing waveforms in words often misses a critical characteristic which becomes clear when seeing the actual waveform. — Martin Thompson
This is indeed puzzling - I'd also very much like to see the scope trace, or even a picture of it if you can get the lighting to work. Does paralleling a second pullup resistor change the voltage? Anything else on the bus? If the I2C master a hardware engine, or bit-banged? — Chris Stratton
@ChrisStratton: Actually the is nothing else on the bus, I mean the rtc device is directly connected to two gpio's pins. The I2C master is then the FPGA (see schematic below) so it is a bit-banged i2c bus! — jrm

Rémi Debord Rémi Debord · Accepted Answer · 2018-03-20T12:09:06

I got the same issue and it was related to the SDA/SCL IOs configuration on the master (open drain shall be used, instead of push pull).

i2c: half amplitude on SDA

1 Answers