ethernetphy
Most of the Ethernet PHY chips (e.g. LAN8720) provide strapping option to control operation.
I am short of IO pins in my uC, and I do not need (AFAIK) to change anything more in the PHY operation.
(1) Can I just not use the MDIO interface (Just PU MDC/MDIO pins)?
(2) If not, What are the "required" operations I need to do with PHY?
Thanks.
As for a 100BASE-TX PHY you can use an approach like this:
You need to physically loop the MDI at the RJ45 jack and then generate test patterns on TX path to the PHY TX path internal logic (i.e. digital and analog logic circuits), receive patterns at RX path---writing and reading the appropriate boundary scan cells (BSC) correspondingly---and compare them (patterns).
In 1000BASE-T case, you need a compatible link partner organizing the remote loop.
Also, the MDI circuits are analog (@Paebbels is right in full here), differential, and they reach out the RJ45 jack pins not direct but thru a transformer with a center frequency of about 30-35 MHz which are unreachable to be generated from JTAG boundary scan even as digital waveforms.
IMO, upgrading the firmware is simpler than using the boundary scan there.
If you need to test a switch IC with integrated PHYs, you need an external test pattern generator/receiver/comparator with internal loopbacks in each tested PHY path. I see no place for and/or advantage of JTAG BS there too.
Assuming you're talking about the pins toward the Ethernet wire, those two Ethernet chips have an "autoreverse" function built-in, that's what "Auto-MDIX" means, i.e. detects if the chip at the other end has Tx/Rx reversed and reconfigures itself automatically.
Best Answer
By definition, "strapping option" is a configuration that is set in hardware by pulling up, down, or no pull on certain pins, which usually takes an effect only at release of hardware reset. This is done to save the number of package pins, which saves on costs. Therefore, there is usually no need to waste GPIO pins on that, unless you plan to do something totally crazy with communication channel.
The operations that are "required" requires you to select certain mode of operation (see Table 3-4 of datasheet), hardwire the straps accordingly, and stick to it. If you want some super-flexibility, you will need to control all straps and hardware reset pin to the PHY.