I2C Signal Integrity: Measurement and Electrical Validation

Overview of I2C Bus:

The I2C’s physical two-wire interface consists of a bi-directional serial clock (SCL) and data (SDA) lines. Each device that is connected to the bus is software-addressable by a unique address and a simple master/slave relationship with the bus exists all the time. I2C is a serial, 8-bit oriented, bi-directional data transfer that can be made at 100Kbits/s in the standard mode, up to 400Kbits/s in the Fast mode, up to 1Mbits/s in fast Mode plus, or up to 3.4Mbits/s in the high-speed mode. On-chip filtering rejects spikes on the bus data line to preserve data integrity.

Phillips Semiconductor (now NXP Semiconductors) has published I2c electrical specifications and I2c protocol specifications since 1982. The recent I2C bus specification and user manual was published in the year 2007. By following the electrical and protocol specification in the I2C document, semiconductor design and manufacturing companies can ensure interoperability of ICs using the I2C Bus.

I2C protocol overview:

Typical data transfer between two ICs using the I2C interface is as shown:

i2c rise time, i2c setup and hold time, i2c signal integrity

All transactions start with START Condition and stop with STOP condition In I2C Bus. These two conditions are controlled by the master IC. The typical I2C frame format has the following contents: START, address, read/write, data followed by ACK/NACK, and STOP condition at end of the operation.

START: A condition where high to low transition of SDA line occurs when SCL is held high. The is initiated by the master IC.

Address: Master sends the slave the 7- bit or 10- bit address of the slave device

Read/write: The slave address is followed by this bit. A ZERO indicates a transmission (write), and a ‘ONE’ indicates a request for READ.

Acknowledge (ACK) and Not Acknowledge (NACK): This takes place after every byte. During this condition, the transmitter releases the SDA line during the acknowledged clock pulse so that the receiver can pull the SDA line LOW, and the SDA line remains stable LOW during the HIGH period of this clock pulse.

When SDA remains HIGH during this 9th clock pulse, this defined as the Not Acknowledgement signal. The master can then regenerate either a STOP condition to abort the transfer or a repeated START condition to start a new transfer.

Data is an integer number of bytes read or written into device.

STOP: A condition during SDA transitions from LOW to HIGH when SCL is held high indicating the end of a transfer of data.

I2C Bus Electrical Measurements:

For successful interoperation of IC using I2C bus, the electrical characteristics of physical layer signals of I2C SCL and SDA signals should be compatibility. The timing between the master and slave devices should be within electrical specifications defined in the I2C Specification by NXP Semiconductor. The electrical measurement list is as follows:

Symbol Parameter Symbol Paramater
fSCL SCL Clock Frequency tr rise time of both SDA and SCL Signals
tHD;STA hold time (repeated) START condition tf fall time of both SDA and SCL signals
tLOW Low Period of the SCL clock tSU; STO set-up time for STOP condition
tHIGH High Period of the SCL clock tBUF bus free time between a STOP and START

condition

tSU;STA set-up time for a repeated START

condition

tVD; DAT data valid time
tHD;DAT Data hold time tVD; ACK data valid acknowledge time
tSU;DAT Data setup time

 

The detailed definitions of each of these measurements are as below:

i2c rise time, i2c setup and hold time, i2c signal integrity

fSCL SCL Clock frequency: Inverse of one cycle period measured at 30% of amplitude of SCL signal. It should be measured at first cycle of after the START condition.

tr rise time of the SCL and SDA signals: time taken by rising edge to reach 70% of the amplitude from 30% of the amplitude of SCL and SDA signals.

tf fall time of the SCL and SDA signals: time taken by the falling edge of the signals to reach 30% of the amplitude  from 70% of the amplitude of SCL and SDA signals

tHD;STA hold time (repeated) START condition: Minimum time the data should be low before SCL is in low state at (repeated) START condition. It is measured as time taken from 30% of the amplitude of SDA at high to low transition to 70% of the amplitude at high to low transition of SCL Signal.

tLOW Low period of the SCL signal: It is minimum low time that should be maintained by SCL signal. It is measured as half period measured at 30% of the amplitude of the SCL signal.

tHIGH High period of the SCL Signal: It is minimum high time that should be maintained by SCL signal. It is measured as half period measured at 70% of the amplitude of SCL signal.

tSU; STO Setup time at STOP condition: It is measured at STOP condition of I2C frame. It is measured as time between 70% of the amplitude at rising edge of SCL signal to 30% of the amplitude of SDA signal at STOP condition.

tSU;STA Setup time for repeated START condition: This measurement is carried out at repeated START condition only. It is time measured between SCL and SDA signal at 70% amplitude of the signals.

tVD; DAT Data Valid time: Measured at every data and clock transition. This is measured with reference to 30% amplitude falling edge of SCL to 70% of rising edge or 30% of the falling edge of SDA signal. The I2C specification maximum allowed data valid time at different I2C speeds.

tVD; ACK data valid acknowledge time: Measured at acknowledgment bit. It is the time from 30% of the falling edge of the eighth clock from start of data to 70% of the ack bit or 30% of the ack bit.

I2C Protocol Electrical Measurement Challenges:

During electrical validation of I2C bus, test engineers need to ensure the I2C bus should comply with electrical parameters of I2C Bus. The challenges faced while electrical validation of I2C bus as follows:

  • Test/Design Engineer must know I2C Protocol behavior at the physical layer of I2C Bus
  • Electrical parameter measurements must be carried out at different protocol state ( example; stop bit, ack bit and so forth)
  • Reference level for each of the measurement changes based on the rising or falling edge of the I2C signal transition
  • The reference level is either 30% or 70% as against normally used reference level of 10% to 90% or 20% to 80%
  • Validation is time-consuming

Overall, measuring I2C electrical measurements demands a very high level of expertise in I2C physical layer behavior, protocol layer, signal acquisition in oscilloscope and I2C Electrical measurement procedures. Due to the complexity of I2C Electrical measurements, the results can be prone to errors.

The solution to I2C Bus Electrical Measurements

Prodigy offers PGY-I2C Electrical Validation and Protocol Decode Software to measure I2C setup and hold time, and I2C rise time. PGY-I2C Electrical Validation and Protocol Decode software offering PHY layer measurements for checking the I2C signal integrity issues.

The PGY-I2C Electrical Validation and Protocol Decode Software offers electrical measurements and protocol decoding as specified in Rev 03, June 2007 I2C Bus specification. Now design and test engineers can automatically make accurate and reliable electrical measurements and decode protocols in PGY-I2C software using data acquired by Tektronix DPO5000, DPO7000, DPO/DSA/MSO70000 Series oscilloscope to reduce the development and test cycle.

PGY-I2C Software runs inside Tektronix oscilloscopes. During the run operation of the application, PGY-I2C sends commands to acquire to SCL and SDA signals of I2C bus. For accurate measurements, the recommended oscilloscope setup is:

  • The signal is at least 5 to 6 six main vertical divisions in the oscilloscope display with the appropriate volts per division.
  • Select the appropriate volts per division to display the signal with at least 5 or 6 main vertical divisions.
  • Select sample rate such that at least 8 to 10 samples present in the rising or falling edge of SCL and SDA signals.
  • Set record length such that at least two I2C frames are captured to make most of I2C signals.

The PGY-I2C software analyzes the acquired data for I2C protocol. The selected measurements are displayed as follows:

i2c rise time, i2c setup and hold time, i2c signal integrity

Electrical Measurement results with limits as specified in I2C standard document

The application makes each of the I2C electrical measurements in every possible I2C protocol state and displays the min, max, and mean values. If the mean value is within the limit specified, the application shows ‘Pass’. But in case, if the mean value is a pass but either the min or max values exceed the limits, applications show‘ pass*’ with an asterisk.

PGY-I2C makes all these measurements in an instant of time addressing all the challenges of I2C electrical measurements, giving accurate and reliable measurements.

Summary:

PGY-I2C Electrical Validation and Protocol Decode software offer the industry’s best I2C Electrical validation and protocol decode software. This software allows design and test engineers to characterize the I2C bus for compliance with I2C bus standard specifications.

PGY-I2C software has leading features ‘Detail View’ to debug the I2C bus at the physical layer and at the protocol layer for electronic system-level development.

PGY-I2C software runs inside industry-leading Tektronix oscilloscopes such as DPO5000, DPO7000, DPO/MSO/DSA70000 to offer the most accurate and reliable solution.