Hello,
I own a NETGEAR WAC104.
I noticed that the OpenWrt wiki says an interesting fact:
The WAC104 is a castrated clone of R6220, with unsoldered WAN port and USB.
According to PCB trace, the USB port goes directly into the processor - but through R79 and R96.
I would say yes. How can you know the values of both resistors?
That is the problem, I do not know.
I tried looking at photos of similar MT7621-based routers, and they all have resistors before USB, but they are unmarked, with no number.
For routers that have schematics available, there is seemingly no resistor in the schematics, but I might be reading them wrong.
Found this
7.3 USB Interface
The MT7621A chip has one set of USB 2.0 interfaces and one set of USB 3.0 interfaces. Please note when designing the carrier board:
USB 2.0 and USB 3.0 interfaces are typically used in Host mode for external
connectivity.
It is recommended to reserve a common mode choke on the signal line to
reduce electromagnetic interference. During debugging, resistors or additional
common mode chokes may be used as needed.
USB 2.0 Board Design and Layout Guidelines
Differential pair intra-pair skew less than 10 mils
Differential pair trace length less than 3 inches
Differential pairs have less than 3 through-holes for layer changes
Differential pair impedance controlled to 90 ohms +/-10%.
Differential pairs are spaced using the 3W rule with other signals
USB 3.0 Board Design and Layout Guidelines
Differential pair intra-pair skew less than 5 mils
Differential pair trace length less than 3 inches
Differential pairs have less than 2 through-holes for layer changes
Differential pair impedance controlled to 90 ohms +/-10%.
Differential pairs are spaced using the 3W rule with other signals
l It is necessary to consider the design of protection circuits on the USB circuit
during the PCB design to meet ESD protection requirements. To avoid the effect
of the protection device on the USB signal and to achieve a good protection
effect, the following principles are recommended for the Coreboard design:
The ESD protection device is recommended to be placed close to the USB
connector port.
The ESD protection device is recommended to be an air 15kV, contact 8kV
device with a response time of less than 1ns.
USB 2.0 has a transmission speed of 480Mbps, so the differential signal is very
sensitive to parasitic capacitance on the line. Therefore, ESD protection devices
with low parasitic capacitance should be selected, with a capacitance of less
than 1pF.
USB 3.0 has a transmission speed of 5Gbps, so the differential signal is very
sensitive to parasitic capacitance on the line. Therefore, ESD protection devices
with low parasitic capacitance should be selected, with a capacitance of less
than 0.4pF.
The 2.2 Ohm resistor in series with the USB signal cannot be modified or
omitted
Very interesting!
I'll try and find some 2.2Ω resistors and test if it works.
Despite that the CPU manages USB3, the R6220 (and so WAC104) is limited to USB2. My guess is that only USB2 pins are connected (only 2 pins data+ data-).
According to this doc, is should be K1 and K2 (page 10). Of course you'll also need VCC and GND.
If you'll ever succeed in your hack, it is worth reporting in the wiki.
Ripped off all the pads.