Embedian, Inc.
42
SMARC-iMX8M Computer on Module User’s Manual v.1.0
2.1.8.2 USB[0:3]_EN_OC# Discussion
The Module
USB[0:3]_EN_OC#
pins are multi‐function Module pins, with a
10k
pull‐up to a 3.3V rail on the Module, an OD driver on the Module, and, if the
OC# (over‐current) monitoring function is implemented on the Carrier, an
OD
driver on the Carrier. The use is as follows:
1) On the Carrier board, for external plug‐in
USB
peripherals (
USB
memory
sticks, cameras, keyboards, mice, etc.)
USB
power distribution is typically
handled by USB power switches such as the Texas Instruments
TPS2052B
or
the
Micrel MIC2026‐1
or similar devices. The Carrier implementation is
more straightforward if the Carrier
USB
power switches have active‐high
power enables and active low open drain
OC#
outputs (as the
TI
and
Micrel
devices referenced do). The USB power switch Enable and
OC#
pins for a
given
USB
channel are tied together on the Carrier. The
USB
power switch
enable pin must function with a low input current. The
TI
and
Micrel
devices referenced above require 1 microampere or less, at a
3.3V
enable
voltage level.
2) The Module drives
USB[0:3]_EN_OC#
low to disable the power delivery to
the
USBx
device.
3) The Module floats
USB[0:3]_EN_OC#
to enable power delivery. The line is
pulled to
3.3V
by the Module pull‐up, enabling the Carrier board
USB
power switch.
4) If there is a
USB
over‐current condition, the Carrier board USB power
switch drives the
USB[0:3]_EN_OC#
line low. This removes the over‐current
condition (by disabling the
USB
switch enable input), and allows Module
software to detect the over‐current condition.
5) The Module software should look for a falling edge interrupt on
USB[0:3]_EN_OC#
, while the port is enabled, to detect the
OC#
condition.
The
OC#
condition will not last long, as the
USB
power switch is disabled
when the switch IC detects the
OC#
condition.
6) If the
USB
power to the port is disabled (
USB[0:3]_EN_OC#
is driven low by
the Module) then the Module software is aware that the port is disabled,
and the low input value on the port does not indicate an over‐current
condition (because the port power is disabled).
Carrier Board
USB
peripherals that are not removable often do not make use
of
USB
power switches with current limiting and over‐current detection. It is
usually deemed un‐necessary for non‐removable devices. In these cases, the