SNLA479 April   2025 DP83822HF , DP83822IF , DP83869HM

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1What is Fiber?
    1. 1.1 Pins Used
    2. 1.2 Hardware Configurations
    3. 1.3 Useful Registers
  5. 2Common Use Cases
  6. 3Summary
  7. 4References

2 Common Use Cases

The following are common occurrences when using the PHY in Fiber mode and potential descriptions of events which can occur if the occurrence happened.

Table 2-1 Common Occurrences Using PHY in Fiber Mode
Event Results
Opposing or local SFP modules are disconnected from board Both PHYs lose link
Both fiber cables disconnected (TX and RX)
Single fiber cable is disconnected Both PHYs lose link due to Far-End Fault (FEF), a functionality where if DUT senses that link has been lost, the DUT can send a FEF signal over the transmitting line so the link partner can also drop link. While FEF is enabled by default, the FEF can have the detection and/or generation functionality disabled through register.
  1. If FEF generation is disabled on DUT, when DUT RX cable is unplugged, PHY does not send FEF to link partner through DUT TX cable. This can cause a scenario where DUT PHY indicates no link, but link partner PHY does indicate link.
  2. If FEF detection is disabled on DUT, when DUT TX cable is unplugged, link partner PHY can send FEF to DUT. However, DUT does not detect FEF and thus not drop link. This can create a scenario where DUT indicates link and link partner indicates no link.
Inverted Signal Detect This can create false-negative or false-positive linkup status. Please confirm polarity of pin, detect prior to plugging in to avoid damage to device or module. If polarity needs to be switched, a register write needs to be written prior to plugging in module.
Mismatching SFP Modules on optical connection (100BASE-FX with 1000BASE-X) PHYs do not link up

Mismatching SFP and PHY speed (822)

 If given a 1G SFP with a matching 1G SFP from link partner side, DP83822 can link up and think the DUT has a fiber link at 100Mbps. Packet throughput was tested successfully. However, if module speeds mismatch, PHYs do not link per above case.
Mismatching SFP and PHY speed (869)

If given a 100Mbps SFP when in 1G Fiber (1000Base-X or 1000M Media Converter), and connected to another DP83869 in same mode, PHY does not link up.

If given a 1000Mbps SFP when in 100M Fiber (100Base-FX or 100M Media Converter) PHYs can link up and have shown valid throughput.