USB Circuit Explanation
Thought for 1 min 3 sec
Series terminations are there to make the driver “look like” the transmission line and to soften edges slightly. That solves two problems at once: reflections (signal integrity) and excessive high‑frequency content (EMI).
Why they help
- Impedance matching at the source: The line (each USB FS single‑ended conductor) has a characteristic impedance. Adding a small resistor in series at the driver makes the driver’s output impedance roughly equal to the line, so the first step launched into the cable/trace is matched. When the far end reflects (due to receiver capacitance/discontinuities), it returns to a source that now absorbs the energy instead of re‑launching it, damping ringing.
- Edge‑rate control: The series resistor with the line/receiver capacitance forms a small RC that slightly lengthens rise/fall times. A gentler edge has less high‑frequency content, which reduces radiated and conducted EMI while staying within the interface’s timing budget.
If you omit them
- Reflections and ringing: Fast edges hit impedance discontinuities and reflect. You’ll see overshoot/undershoot and multiple threshold crossings, which can close the eye, add jitter, and cause intermittent USB FS errors.
- EMI spikes: Sharper edges and undamped ringing increase spectral peaks, making emissions and susceptibility worse and risking compliance failures.
- Stress on clamps/ESD structures: Large overshoot/undershoot can forward‑bias on‑chip ESD diodes, distorting the waveform and increasing power dissipation.
- Tighter placement tolerance: Without damping, small layout stubs or via transitions have a bigger negative effect.
Practical guidance
- Place one small series resistor in each data line right at the transmitter pins; keep the trace length before the resistor minimal.
- Use matched values on the differential pair and maintain tight coupling/controlled impedance along the route.
- Some MCUs already include internal output resistance; in that case the external resistors may be reduced or omitted—verify in the device datasheet and USB layout guidelines.
Ask about USB FS values
Check MCU output impedance
Placement guidance
When can omit
EMI impact explanation
