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Why Series Termination Resistors Are Required on Communication Lines – KBA225247

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Why Series Termination Resistors Are Required on Communication Lines – KBA225247

Author: BragadeeshV_41           Version: **

Translation - Japanese: 通信回線で直列終端抵抗が必要な理由– KBA225247 - Community Translated (JA)

The need for series resistor can be explained with respect to the maximum power transfer theorem. The law states that to transfer maximum amount of power from the source to a load, the load impedance should match the source impedance.

While transmitting data from one device to another using SPI or I2C, the two devices are connected via a transmission line. For maximum power to be transferred, the transmission line impedance should match the source and load impedance. If the impedances aren’t matched, maximum power will not be delivered. This means that the load does not absorb all the power that is transmitted and will reflect back towards the source and the therefore, the signal is lost. To prevent the loss of signal, series terminating resistors are added for impedance matching.

Series resistors are recommended only in the case of very-high-frequency communication (more than 400 kHz) or when the communication line traces are over a large trace distance. A trace is considered as a long trace if the time it takes for a signal to travel down a trace and reflect back is significantly longer than the rise time of the signal.

Series resistors can also be used for protection against high-voltage spikes on communication lines owing to high frequency. Also, the voltage swing on the bus reduces. However, a high value of the series resistor would slow down the rising edges of the signal, which makes them susceptible to noise. Therefore, the right value for the resistors should be chosen to reduce high-voltage spikes in the communication lines without causing it to slow down the signal drastically.

I2C Communication:

In the case of a I2C single Master – single Slave configuration, a series resistor is recommended to be placed close to the Master device. For all other configurations, series resistors are placed close to each of the Master / Slave device connected to the bus. Refer to the I2C Bus Specification (https://www.nxp.com/docs/en/user-guide/UM10204.pdf) to choose the right value for series resistors and corresponding pull-up resistors.

SPI Communication:

In the case of SPI communication, for the Master device, place the series resistors near the Master In Slave Out (MISO) pin. For the Slave device, place the series resistor near the Master Out Slave In (MOSI) pin.

CapSense® Systems:

In the case of design involving CapSense, a series resistor of 330 ohms is recommended to be added to communication lines such as I2C and SPI. See the “Series Resistors on Digital Communication Lines” section of the PSoC 4 and 6 CapSense Design Guide (http://www.cypress.com/file/46081/download).

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