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17.4.2
Controlling the D+ Terminating Resistor on the Board
This section describes those points related to the USB function, which must be noted
when controlling connection or cutting of the on-board terminating resistor for the USB
D+ signal. This control is performed to reserve the time required for the application
(firmware) initialization routine.
■ Controlling the D+ Terminating Resister on the Board
The hub detects connection of a function device to a downstream USB port when either of the
following conditions is met:
•
The power of a function device already connected to the USB is turned on.
•
A function device whose power has already been turned on is connected to the USB.
The USB specifications require a function device to be able to accept a transaction within 100 ms
after the hub has detected its connection to a USB port.
If either of the above conditions for the hub's detection of the connection of a function device to a
USB port is met, this macro program must execute a sequence to set up the end point buffer after
reset processing.
If the firmware requires 100 ms or more for the initialization routine, including reset and setup of
the end point buffer, there is a method to defer the detection of the connection. The method is to
cut the terminating resistor for the USB D+ signal to temporarily prevent the hub from detecting a
port connection . When using this method, take the following precautions for the macro program:
Notes:
• Setup of the end point buffer must be started after the hardware and software are released from
the reset status.
• Control to connect the terminating resistor must be started after end point buffer setup by the
firmware is completed.
Moment of power-on or USB connection
USB D+ terminating
resistor control
1: Connected
Software reset
(Negative logic)
Hardware reset
(Negative logic)
The hub does not recognize a connection
although the device is physically
connected to the USB.
Period in which end point buffer
setup is enabled
CHAPTER 17 USB FUNCTION
The hub detects
connection to the USB.
441
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