pin and some applications will require they be implemented in software via a GPIO. Software that implements
these signals is available in the USB software library.
17.1.1.3 VBus Recommendations
Most applications do not need to monitor V
Bus
. Because of this, a dedicated V
Bus
monitoring pin was not
included on this microcontroller. If you are designing a bus-powered device application or an embedded host
application, you do NOT need to monitor V
Bus
. If you are designing a self-powered device, you will need to
actively monitor the state of the V
Bus
pin in order to ensure compliance with the USB specification. In Section
7.1.5 and Section 7.2.1 of the USB Specification Revision 2.0
™
it is stated respectively that:
• "The voltage source on the [speed identification] pull-up resistor must be derived from or controlled by the
power supplied on the USB cable such that when V
BUS
is removed, the pull-up resistor does not supply
current on the data line to which it is attached.
• When V
BUS
is removed, the device must remove power from the D+/D- pull-up resistor within 10 seconds.
• Later in the timing tables (Section 7.3.2) of the USB Specification 2.0 it is also stated that the D+/D- pull-up
resistor should be applied within 100ms of V
Bus
reaching a valid level."
Meeting the above specification is easy because of the slow timing requirements. In this chapter we will discuss
the hardware part of the V
Bus
monitoring solution. The corresponding software will be discussed briefly, but for
examples and an explanation, please consult the USB software guide.
The pins of this microcontroller are not 5V tolerant, and because of this, the V
Bus
signal cannot be directly
connected to a GPIO pin. Directly connecting 5V to a pin of the microcontroller will destroy the I/O buffer of
the pin and possibly more of the chip. The most cost-effective way of making any pin capable of reading a 5V
input is to use a series resistance in conjunction with the ESD diode clamps already present inside the device
on every pin. It is recommended to use a 100kΩ series resistor between the V
Bus
signal and the pin chosen to
monitor it. A diagram of this setup is shown in
In the below diagram, if V
Bus
is above or below 3.3V and 0V, respectively, one of the ESD clamp diodes will
be forward-biased, allowing current to flow through the 100KΩ resistor. The purpose of the diode clamps is to
protect the pins of the microcontroller from very short overvoltage spikes of a high magnitude. They do this by
clamping the voltage excursion to one of the supply rails. We are effectively requiring the ESD clamps to do the
same thing they were designed to do, but instead of a short high magnitude pulse, we are giving them a long low
magnitude static value via the 100kΩ resistor.
VBUS
D+
D–
GND
GND
f2806xU
GND
100 k
GPIOx
USB-DP/GPIO26
USB-DM/GPIO27
P$1
P$3
P$2
P$4
+3V3
Figure 17-2. USB Scheme
Any pin that has digital input/output functionality could potentially be used to monitor V
Bus
, but the use of an
interrupt-capable GPIO is recommended. A pin that does not have external interrupt capability may also be
used, but the input state of the pin must be polled periodically by the application software to ensure appropriate
action is taken whenever V
Bus
is applied or removed. If an interrupt-capable GPIO is chosen, it should be
configured to generate an interrupt on both the rising and falling edge. More information on external interrupts
can be found in the
System Control and Interrupts
chapter. Example code that implements V
Bus
monitoring using
external interrupts and takes the appropriate actions is documented in the USB Software Guide and can be
found in the associated USB software package.
Universal Serial Bus (USB) Controller
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
1057
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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