Measuring and analyzing I²C signals

Introduction

I2C (Inter-Integrated Circuit) is a serial bus, that is used to exchange data with low-speed peripherals. I2C uses two bi-directional lines, serial data (SDA) and serial clock (SCL). Typical voltages used are +5 V or +3.3 V. The most common I2C bus modes are the 400 kbit/s fast mode, the 100 kbit/s standard mode and the 10 kbit/s low-speed mode. A high speed mode of 3.4 Mbit/s is also available.

I2C uses a 7 bit or 10 bit address, depending on the device, with 16 reserved addresses, which means that a maximum of 1008 nodes can communicate on the same bus. Two types of nodes are possible: master and slave. I2C devices use a dedicated protocol to communicate with each other.

When devices on a I2C bus are not communicating properly, something must be wrong and the error has to be found. Measuring the electrical parameters of the two signals in the I2C bus can be done easily with an oscilloscope. But, when the electrical parameters are all OK, the signals have to be analyzed, to check if the communication protocol is implemented properly.

Requirements

Measuring

To measure I2C signals, a measuring instrument with at least two channels is required. The maximum frequency on the I2C bus depends on the bus type, the instrument must sample at at least twice the maximum speed on the bus, but preferable five to ten times higher, on both channels. For a 100 kbit/s bus that would mean a minimum sampling frequency of at least 1 MHz on both channels. I2C uses either 3.3 V or 5 V, so the instrument should be able to measure voltages between 0 and 5 V. Since data transfers can be long, a long record length is preferred to capture the communication.

The Handyscope HS3, Handyscope HS4, Handyscope HS4 DIFF, Handyscope HS5 and the Handyscope HS6 DIFF are suitable instruments to measure I2C signals.

Analyzing

To examine the communication protocol in the measured signals, the various pulses on both signals have to be examined and their sequences have to be converted in I2C commands.

The TiePie engineering Multi Channel oscilloscope software is capable of analyzing the I2C communication protocol, using the I2C analyzer.

In this example, a Handyscope HS4 is used to measure I2C signals on an I2C EEPROM that is being accessed.

Setting up the hardware

First the Handyscope HS4 is connected to the computer and the Multi Channel oscilloscope software is started.

Now connect Ch1 to the Serial Clock signal of the I2C bus and Ch2 to the Serial Data signal of the I2C bus. The ground terminals of the two input channels must be connected to the same ground as the I2C bus uses.

Setting up the software

Setting up the input channels

Since only two channels are used to measure the I2C signals, Ch3 and Ch4 of the Handyscope HS4 are removed from the screen.

We are using Ch1 to measure the Serial Clock signal (SCL) and Ch2 to measure the Serial Data signal (SDA). To simplify recognition of the signals, they can be given a descriptive name (alias). To change the alias of a channel, right-click the channel in the object tree and select Set alias... and enter the required alias. Give Ch1 the alias "SCL" and Ch2 the alias "SDA".

I²C signals lie between 0 V and either +3.3 V or +5 V. Therefore, set the channel input coupling of both channels to "DC" and set the input sensitivity of both channels to "8 V" full scale. That way both signal levels can be measured properly.

Setting up the time base

The I2C signals in our example are measured on a 100 kbit/s bus, and can therefore be up to 100 kHz in frequency, which means that the minimum required sampling frequency must be 200 kHz. However, this results in only 2 samples per period, barely enough to recognize the signal but not enough to analyze it properly. Therefore, set the sampling frequency to at least 1 MHz.

Since I2C communication can take several milliseconds, a long record length is preferred, to capture as much as possible of the communication. In this example, a relative short communication is measured, therefore the record length is set to 20000 samples. When a longer communication is expected, set the record length to a longer value, e.g. 100000 samples.

Setting up the trigger

Both SCL and SDA are high in idle state. When a communication starts, first the SDA line is made low. Therefore, select Ch2 as trigger source, set the trigger mode to "falling edge" and set the trigger level to a value half way the signal, e.g. 1.5 V. Set the trigger hysteresis to e.g. 0.5 V. A trigger will now occur when Ch2 measures a falling edge, where the signal is initially above +2 V and drops below 1.5 V.

measuring I²C signals

Setting up the I2C analyzer

To analyze the I2C signals, the I2C analyzer sink is used. Create one by right-clicking Sinks in the object tree. This also opens an additional window, which will display the analyzed I2C communication.

The I2C analyzer requires two sources to be connected, the first source that is connected is treated as SCL, the second signal is treated as SDA. First drag Ch1 on the I2C analyzer and then drag Ch2 on the I2C analyzer.

The I2C analyzer output window will now display which source is connected to the SCL input and which source is connected to the SDA input of the analyzer.

I²C analyzer

When the inputs are connected to the wrong sources, a click on the Swap inputs button SwapBtn will swap the inputs.

The I2C analyzer can analyze both 3.3 V I2C buses and 5 V I2C buses. To set the bus voltage, right-click the I2C analyzer sink in the object tree, select Voltage and then select either 3.3 V or 5 V. In our example we measure on a 3.3 V bus.

Ready to measure

Now both input channels and I2C analyzer are configured, a measurement can be performed and analyzed.

The I2C signals shown in the screen shot above are from communication with an EEPROM. They are converted by the I2C analyzer into the following commands:

measuring I²C eeprom

All read and written data displayed by the I2C analyzer is shown in hexadecimal format, only addresses are prefixed with 0x.

To clear the I2C output window for a new measurement, press the Clear text button ClearBtn.

When a block of communication is longer than the I2C output window, it is not possible to see all text that is appended. The window has an auto scroll function, which always makes sure the bottom lines are visible. To toggle this function, click the Auto scroll button ScrollBtn.

To save the contents of the output window to a file, press the Save button SaveBtn.

Setting address aliases

The I2C protocol uses addresses to access the various devices on the bus. To simplify reading the decoded I2C communication, the addresses in the output window can be replaced by descriptive names (aliases). Click on the Aliases tab in the I2C analyzer output window to set the aliases for the addresses that occur in the decoded communication.

Setting aliases

Aliases for addresses can be added by clicking the Add Alias button AliasBtn and edited with the Edit alias button EditBtn. Often used aliases can be saved to disk (SaveBtn) and restored from disk (OpenBtn).

Once address aliases are set, all next measurements will display the entered aliases.

measuring I²C eeprom

Cleaning dirty signals

In industrial environments, the measured signals can be very `dirty`, causing the I2C analyzer to have problems decoding the communication properly. Cleaning the signals may improve the ability of the I2C analyzer to decode the signals properly.