Can the ADUM4160 be used for USB-IF standard host isolation?
The ADuM4160 USB isolator is designed to isolate a peripheral port and keep the
port compatible with the USB-IF standard. It does that job very well. It was
not intended for host isolation.
That being said, because this part was intended to drop into the USB line it
has a peripheral type port and a host type port to allow it to interface with
the peripheral port of the end point device. The downstream host port is side 2
of the device and it contains the proper pull downs and drive strength to
provide host type communications. The only thing holding it back from isolating
a host is that full compliance to the USB spec requires that a host auto-detect
the speed of any peripheral plugged into it. The ADuM4160 was designed to
operate at a single speed, set by hard wiring pins SPU and SPD. So it can be
set up and will communicate as a host isolator at either low or full speed, but
it will be a fixed speed device however it is set, and so does not meet all of
the requirements of the USB-IF standard. For many customers this is adequate
for their job and they use it in this way.
So the ADUM4160 will function at either low speed or full speed USB (but only
at whichever speed has been set by hardwiring of the SPU and SPD pins. The
ADUM4160 does not support high speed USB and so it fails to pass high speed USB
signals because it blocks handshaking signals that are used to negotiate that
The circuit note CN0158 describes a circuit implementation to isolate a host
device. The hub chip chosen in this circuit note supports high speed operation,
but this mode is not allowed by the normal operation of the ADuM4160. The
upstream side of the hub controller can be thought of as a standard fixed speed
peripheral port that can be easily isolated with the ADuM4160, whereas the
speed of the downstream ports is handled by the hub controller. The hub
controller converts peripherals of different speeds to match the upstream port
speed so the hub solves the problem of adjusting the speed of the downstream
ports to match the hardwired (low or full speed) setting on the ADUM4160.