In Serial Peripheral Interface (SPI) communication, the clock signal (SCLK) is crucial for the proper timing and synchronization of data transfer between the master and slave devices. In this blog, we will take a closer look into the clock polarity, phase, the rising/falling edges, as well as the different SPI modes, and how these influence the configuration of the clock signal and how it operates.

Signals Used to Facilitate SPI Communication

The SPI protocol is a full-duplex communication protocol commonly used to facilitate data transmission between embedded system devices. A standard SPI connection involves a master connected to one or more slaves via four lines: Serial Clock (SCLK), Master Out Slave In (MOSI), Master In Slave Out (MISO), and Slave Select (SS). The SCLK, MOSI, and MISO signals can be shared amongst the slave devices, while each slave has a unique SS line to enable communication.

The SCLK is essential for synchronizing data between the master and slave devices. This clock signal is generated by the master and is used by both the master and slave to synchronize the data transmission.

What is Clock Polarity and Clock Phase of the Clock Signal?

SPI communication relies on two parameters to set the timing and format of data exchange between devices: clock polarity (CPOL) and clock phase (CPHA),

CPOL defines the idle state of the clock signal, which is when there is no data being transferred between devices. In SPI communication, the clock can either be idle low (held at a low voltage level) or idle high (held at a high voltage level), depending on the CPOL setting. For instance:

• If CPOL is 0, the clock idles at a low level.
• If CPOL is 1, the clock idles at a high level.

These idle states help establish a baseline for communication timing between SPI devices. Devices must agree on the idle state (low or high) based on the CPOL setting to ensure proper synchronization and data transfer reliability.

The CPHA setting establishes when data is sampled and shifted relative to the clock signal's edges. With CPHA = 0, data is sampled on the leading edge and shifted on the trailing edge, whereas with CPHA = 1, data is sampled on the trailing edge and shifting on the leading edge.

Clock Edges: Rising Edge and Falling Edge

Clock edges refer to the rising and falling edges of the clock signal. The rising or falling edge determines the exact moment when data bits are read or written.

The Rising Edge is the transition of the clock signal from a low state to a high state. Depending on the SPI mode, the rising edge can be used to either sample or shift the data.

The Falling Edge is the transition of the clock signal from a high state to a low state. Similar to the rising edge, the falling edge can be used for sampling or shifting data depending on the SPI mode.

The 4 SPI Modes

There are four possible clocking configurations of CPOL and CPHA, often referred to as SPI modes:

• Mode 0 (CPOL=0, CPHA=0): CLK idle state = low, data sampled on rising edge and shifted on falling edge.
• Mode 1 (CPOL=0, CPHA=1): CLK idle state = low, data sampled on the falling edge and shifted on the rising edge.
• Mode 2 (CPOL=1, CPHA=0): CLK idle state = high, data sampled on the falling edge and shifted on the rising edge.
• Mode 3 (CPOL=1, CPHA=1): CLK idle state = high, data sampled on the rising edge and shifted on the falling edge.

Below is a diagram of the Clock Polarities and Clock Phases of SPI Modes:

SPI modes ensure compatibility between SPI master devices (like microcontrollers) and slave devices (such as sensors, memory chips) that adhere to specific timing and data transfer protocols.

SPI Debugging and Development Tools from Total Phase

Configuring a proper clock signal in SPI communication is essential to maintain precise timing and ensure data integrity and synchronization between devices. Misalignment of clock timing and configuration can lead to data corruption or communication errors.

To help engineers gain visibility into their SPI systems and bus data, Total Phase offers various debugging and development tools. We offer a line of I2C/SPI host adapters that fit different project requirements as well as I2C and SPI sniffers, which all can be interfaced through our free software or API. Take a look at our I2C/SPI Product Guide for more information on these tools and their features.

Debugging SPI Communication in Real Time

The Data Center Software is our industry-leading bus monitoring software that provides detailed insight into various protocols, including I2C, SPI, USB, CAN, and eSPI.

For SPI systems, users can specify the sampling edge of the data frame, or SPI mode, within the Data Center Software so the software can properly parse SPI data.

The Transaction window displays all the transactions that were captured on a serial bus in real time, as well as bus events or capture meta-information such as when the capture began or ended. When a transaction is selected, the byte content and/or timing data of that transaction is displayed in the Details window.

SPI transactions in Transaction window in Data Center Software

The Details Window provides lower-level detailed information about a specific transaction, including data (MISO, MOSI) and bit timing details.

Data tabs in Data Center Software displaying MOSI/MISO data

Timing tab in Data Center Software

Testing SPI Systems

For programming or testing SPI devices, Total Phase offers our Control Center Serial Software. Users can configure SPI master or SPI slave modes on their host adapter and send/receive messages to test systems.

The SPI Control module allows users to configure various data frame parameters for master and slave devices including, Polarity and Phase and Bit Order, to specify how to clock the bits that are sent and received on the adapter SPI interface.

Additionally, the Cheetah GUI functionality within SPI Master Mode provides access to the SPI functions of the Cheetah SPI Host Adapter and allows users to specify between the four modes of SPI.

SPI Master Mode in Control Center Serial Software

Conclusion

In conclusion, understanding SPI clock polarity, phase, clock edges, and the various SPI modes is essential for developing and debugging SPI devices effectively. With Total Phase tools, engineers can get specific insight into the data and timing of SPI transactions and test their SPI master or slave devices with an easy-to-use interface. Our tools make it possible for engineers to optimize their SPI communication for robust device performance.

For more information on our SPI tools and how they can be used for your specific application, please contact us at sales@totalphase.com.