When a microcontroller or FPGA sends SPI control signals to a phase-locked loop (PLL) chip, it often requires a series connection of a 22 ohm or 33 ohm resistor. What is the reasoning behind this?
Impedance discontinuities can cause electromagnetic wave reflections, which in turn can lead to signal distortion. Therefore, impedance matching is often involved in high-speed digital circuit design. However, this matching differs operationally from RF matching.
In RF matching, inductors and capacitors are typically used to reduce reflections, while in digital circuits, resistors are generally used for matching.
In digital circuits, source matching is one of the matching methods. The aforementioned series resistor in the signal path is a typical example of source matching. Assuming the output resistance of the digital circuit is Zd and the characteristic impedance of the transmission line is Z0, the series resistor R can be calculated as R = Z0 – Zd.
It has been mentioned in forums (1) that the output resistance of digital circuits can be calculated using Zo = (Vdd – VOH) / IOH and Zo = VOL / IOL. However, in practice, this is not always ideal.
As noted in (2), even for the same model of device, the values can vary significantly.
Therefore, in practical applications, it is advisable to leave a space in the circuit to change the resistor values while using an oscilloscope to test when the ringing in the waveform disappears.
References:
(1)https://www.physicsforums.com/threads/how-to-determine-the-output-impedance-of-cmos-gates.433643/
(2)https://forums.xilinx.com/t5/General-Technical-Discussion/Resistor-termination-for-FPGA-to-SPI/td-p/160122
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