In circuits used for SPI bus communication with peripheral devices, a small resistor with a resistance of tens of ohms is typically connected in series with the signal line, as shown in Figure 1. This design achieves the following functions:
1. Impedance matching. When SPI signal lines are long or the load capacitance is large, the signal may be reflected at the end of the transmission line, causing waveform oscillation (ringing) or overshoot/undershoot. The series resistor acts as an impedance match at the source end (usually close to the master end), absorbing reflected energy and reducing signal integrity issues. This is particularly critical in high-speed SPI (e.g., tens of MHz) or long traces.
2. Reduce electromagnetic interference (EMI). SPI communication often operates at high speeds. The series resistor, along with the capacitance of the line and the load capacitance, forms an RC circuit. This circuit structure helps slow down the rising and falling edges of the signal, thereby preventing overshoot. This has a positive effect on EMI suppression, especially in high-speed circuits.
3. Current limiting and device protection. The resistor limits current flow in the event of an accidental short circuit (e.g., due to wiring errors), preventing damage to the master or slave device I/O ports.
4. Optimize debugging. During debugging, it's common to use an oscilloscope to capture waveforms. Connecting a resistor in series with the SPI signal line makes it easier to observe and debug the signal waveform using an oscilloscope, improving debugging efficiency.
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