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11. Serial Data Corruption Errors

11.1 Impedance Matching

Problem 25: There is frequent to intermittent data corruption between the MCU and target IC.

Signal Integrity issues or reflections can occur due to fast signal rise and fall times, and impedance mismatches

between source and target devices. It follows that the higher the signal frequency

, then the rise and fall times are

faster, see the Signal Integrity figure below.

A general guideline to determine when the transmission line termination approach is necessary is as follows:

• Terminate the transmission line in its characteristic impedance when the one-way propagation delay of the PCB

trace is equal to or greater than one-half the applied signal rise/fall time (whichever edge is faster)

Since most designers do not know the PCB trace propagation delay, the following procedure should be used:

• Check that all high-speed signals in excess of ~15 MHz are terminated with a series resistor at the point of

origin, (i.e., source of the signal), or in the case of bi-directional signals, at all device output pins on the bus

To determine which termination resistor value to use based on the user’s PCB design specifications, calculate the line

impedance at: https://www.eeweb.com/tools/microstrip-impedance = Trace Impedance

Equation 3: PCB signal series termination resistor(s) value = {[(VDD-VOH(min)) /IOH(max)] - Trace Impedance}

Table 11-1. Typical PCB Trace Characteristics Illustration Only

Trace Width PCB Height between layers Trace Thickness PCB Substrate Dielectric Trace Impedance

0.175 mm 113 µm 1.41 mils (1 oz Cu) 4 54.6 Ohms

For example,

PCB signal series termination resistor(s) value = {[(VDD-VOH(min)) /IOH(max)] - T

race Impedance}

= {[(3.3-2.4) / 10 ma] – 54.6 Ohms}

= 35.4 Ohms (rounding up or down to the nearest standard resistor value)

= 33 Ohms

Note: Too much termination resistance will impact the slew rate and speed of the signal to the point that the signal

no longer meets specified timing requirements.

Serial Data Corruption Errors

echnology Inc.

and its subsidiaries

Manual

DS70005439B-page 33

Microchip Technology SAM Series User Manual

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