Support Question of the Week: How Can I Interface the Beagle SPI Protocol Analyzer to a High Impedance SPI Signal Line and Resolve the Signal Noise?

I have a question about interfacing the BeagleTM IC2/SPI Protocol Analyzer with my setup. I am using the ST Microelectronics STM32F071CB as the SPI master and the Nuvoton ISD2115 as the SPI slave device. The clock is set to 750Khz and the SPI bus is operating in Mode 3. My Data Center acquisition is set up for SPI at 10MHz. Between the two devices are 470 ohm resistors on the SPI signal lines. They are placed close to the slave device - the Beagle connects to the STM device, but the 470 ohms is between the Beagle and the Nuvoton target device. Both devices and the SPI signal lines operate at 3.3V.

The Beagle SPI protocol analyzer and the Data CenterTM Software are working fine, but the behavior is strange when the Beagle analyzer is connected to the target device - the symptoms are similar to when there is signal noise on an SPI signal line.

What causes this chatter?

  • Is this setup okay?
  • What is the input impedance of the Beagle SPI sniffer device?
  • What is the recommended circuitry so that the Beagle can monitor the SPI signals?
  • Also, for another project, does the Beagle analyzer work with 5V signals as well as 3.3V signals?

Thanks for your questions! The Beagle I2C/SPI Protocol Analyzer is compatible with 3.3V or 5V signal levels for I2C and SPI devices. The input characteristics of the Beagle I2C/SPI Protocol Analyzer are approximately 15pF parasitic capacitance and 10uA leakage current, which calculates to a minimum of 330k ohms. In actual practice, it is more like 1M ohms.

The Beagle I2C/SPI Protocol Analyzer supports 3.3V and 5V signals. Figure 1: Beagle I2C/SPI Protocol Analyzer


We have two recommendations for your setup:

Connecting the Beagle Directly
It is usually best to connect the Beagle I2C/SPI Protocol Analyzer directly to the I2C/SPI circuit while considering the above mentioned AC characteristics.

Using a Level Shifter Board with the SPI Circuitry
However, in your case, the output impedance of the target system may be too high to work directly with the Beagle analyzer. The Beagle analyzer has a bus-keeper circuit that uses a 50k pull-up/pull-down resistor, and your target system may not interface properly with its drivers. However, this can be resolved by using our Level Shifter Board between the analyzer and the target system or by using pull-down/ pull-up resistors. Details about using the Level Shifter Board are provided in the Level Shifter Board User Manual and the knowledge base article Programming I2C EEPROM Using Aardvark Adapter, Level Shifter Board and Control Center. You can also contact us for additional information about configuring the Level Shifter Board for your system. In addition to high impedance, the Level Shifter Board can be used to support interfacing our host adapters and protocol analyzers to other signal levels, 1.2V, 1.5V, 1.8V, 2.5V, 3.0V, and 3.3V.


Use the  Level Shifter Board to interface the Beagle I2C Protocol Analyzer to resolve signal noise cause by impedance mismatch. Figure 2: Level Shifter Board


For additional information about Protocol Analyzers, Accessory Boards and other Total Phase products, please refer to the following documents:

We hope this answers your question. If you have other questions about our Total Phase products, feel free to email us at or submit a request for technical support.

Leave a Reply