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1. General recommendations[edit source]
At IC level, Eye Diagram is characterized with a specific reference resistor between USB_RREF and ground.
Verify first that USB_RREF resistor is correctly connected and has proper value and precision (3 kΩ 1%)
2. Eye Diagram[edit source]
2.1. Common mode troubleshooting[edit source]
- Hardware root causes to investigate
- Mismatch between D+ and D- (in terms of length and/or parallelism)
- Check layout and add common mode filter (e.g. ECMF02-2AMX6[1])
2.2. Jitter troubleshooting[edit source]
- Hardware root causes to investigate
- Too much capacitance (component or PCB) on D+/D- lines
- USB Power/GND instability
- Check decoupling / Check grounding
- Software potential tuning
Only fine tuning available. High jitter has an HW root cause in most cases.
- USBPHYC_TUNEx.HSDRVRFRED
- 0: increase rising edge but add jitter
- USBPHYC_TUNEx.INCURREN/.INCURRINT
- Reduce current increase with .INCURREN=0 or .INCURRINT=0
- USBPHYC_TUNEx.HSDRVCHKITRM
- Reduce the increase of the main level
2.3. Level attenuation or Signal-to-Noise troubleshooting[edit source]
- Hardware root causes to investigate
- too much serial resistance and capacitance (component or PCB) on D+/D- lines
- USB Power/GND instability
- Check decoupling / Check grounding
- Software potential tuning
- USBPHYC_TUNEx.HSDRVCHKITRM
- Higher the value, higher will be the level of signal.
- But noise and ripple can appear
- USBPHYC_TUNEx.HSDRVCURINCR/.HSDRVDCLEV
- Higher the value, higher will be the margin for the level.
- But jitter in the rising edge and noise can appear
2.4. Rising edge or Falling edge troubleshooting[edit source]
- Hardware root causes to investigate
- too much serial resistance and capacitance (component or PCB) on D+/D- lines
- Impedance mismatch : check characteristic impedance of lines, stubs…
- Software potential tuning
- USBPHYC_TUNEx.HSDRVRFRED
- 0 improve rising edge
- 1 slow-down rising edge.
- Potential impact on jitter
- USBPHYC_TUNEx.HSDRVCHKITRM
- Depending on level, slope is increased/decreased
- USBPHYC_TUNEx.HSDRVSLEW
- 0 normal rising edge
- 1 reduce rising edge.
- Useful to reduce a hill shape ripple on top of rising/falling edge
- USBPHYC_TUNEx.HSDRVCURINCR/.HSDRVDCLEV
- Improve slope speed.
- But jitter in the rising edge and noise can appear
2.5. Ripple troubleshooting[edit source]
- Hardware root causes to investigate
- Impedance mismatch : check characteristic impedance of lines, stubs…
- Software potential tuning
- USBPHYC_TUNEx.HSDRVCHKZTRM[1:0]
- 00: Usually open the eye but can create some jitter and ripple
- 11: Close the eye but can reduce jitter and ripple
- USBPHYC_TUNEx.HSDRVSLEW
- 0 normal rising edge
- 1 reduce rising edge.
- Useful to reduce a hill shape ripple on top of rising/falling edge
- References: