Application Note
USB OTG Like Connector Implementation
AT91SAM ARM-based Microcontrollers
1.
Scope
The USB OTG (On-The-Go) is an interface to communicate with portable USB devices.
In the SAM9x5 Series (SAM9G15/G25/G35/X25/X35) and SAMA5D3x products, the USB host
and device share one transceiver. The shared USB transceiver can be controlled either by the
USB Host Port Controller or by the USB Device Controller. This configuration allows to emulate
a "USB OTG like" connector. The purpose of this document is to help the developers to
implement this kind of USB OTG.
2.
References
[1] Universal Serial Bus Specification, Rev 2.0, April 27, 2000
[2] On-The-Go Supplement to the USB 2.0 Specification, Rev 1.0a, June 24, 2003
[3] http://www.at91.com/linux4sam/bin/view/Linux4SAM/GettingStarted_9x5
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3.
USB OTG
3.1
USB OTG Purpose
The USB OTG is a USB controller that can be auto-confabulated to host or device according to ID pin status. It means
that one mini footprint receptacle can be configured to host or device.
3.2
USB OTG Features
An On-The-Go device must include the following features and characteristics:
3.3
z
A limited Host capability
z
Full-speed operation as a peripheral (high-speed optional)
z
Full-speed support as a host (low-speed and high-speed optional)
z
Targeted Peripheral List
z
Session Request Protocol
z
Host Negotiation Protocol
z
One, and only one Mini-AB receptacle
z
Minimum 8 mA output on VBUS
z
Means for communicating messages to the user
USB OTG Plug Connection
The ID pin on a Mini-A plug shall be connected to the GND pin; the ID pin on a Mini-B plug is not connected (open drain)
or is connected to ground by a resistance greater than 100 K.
Figure 3-1. USB OTG Plug Connection
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3.4
USB OTG Connectors
The OTG supplement defines the following additional connectors:
z
Mini-A plug and receptacle
z
Mini-AB receptacle
Figure 3-2. USB OTG Connectors
USB OTG Host Negotiation Protocol
The Host Negotiation Protocol (HNP) allows the host function to be transferred between two directly connected OTG
devices and eliminates the need for a user to switch the cable connections in order to allow a change in control of
communications between the devices. HNP will typically be initiated in response to an input from the user or an
application on the OTG B-device. HNP may only be implemented through the Micro-AB receptacle on a device. The Adevice is always responsible for powering the USB interface regardless of whether it is acting in host or peripheral role.
To understand the need for the HNP and host/peripheral role reversal, the example in Figure 3-3 shows two OTG
devices, a PDA and a printer. The PDA has a printer driver inside. The two devices are connected with the new OTG
cable as shown, making the printer the default host (A-Device) and the PDA the default peripheral (B-Device). But this
setup is backwards. The PDA, which has the printer driver, needs to act as USB host to the printer, which contains no
driver. Rather than bothering the user to reverse the cable, HNP allows the devices’ roles to reverse automatically and
silently.
mAB
mA
mB
Figure 3-3. USB OTG Connection
mAB
3.5
PRINTER
PRINTER
DRIVER
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3.6
USB OTG Session Request Protocol
In order to conserve power, the OTG supplement allows an A-device to leave VBUS turned off when the bus is not being
used. If the B-device wants to use the bus when VBUS is turned off, then it requires the A-device to supply power on VBUS.
For this reason, the OTG supplement defines the Session Request Protocol (SRP).
An OTG session is defined as the time that the A-Device is furnishing VBUS power (Note: the A-Device always supplies
VBUS power, even if it is functioning as a peripheral due to HNP). The A-Device can end a session by turning off VBUS to
conserver power, a very important requirement in a battery-powered device such as a cell phone.
mAB
CELL PHONE
3.7
mAB
SRP PULSE
mA
VBUS
mB
Figure 3-4. USB OTG Session Request Protocol (SRP)
VBUS
5V
8mA
CELL PHONE
USB OTG Electrical Characteristics
Table 3-1.
USB OTG Electrical Characteristics
Parameter
Conditions
Min
Max
Units
A-device Output Voltage
0≤IVBUS≤IA_VBUS_OUT<100 mA
4.4
5.25
V
100
ms
0.2
V
CLOAD=10 µF
VBUS Rise Time
IVBUS=rated load
0V<VBUS<4.4V
A-device Leakage Voltage
A-device Output Current
4.4V≤VBUS≤5.25V
B-device (SRP capable) to OTG
Device Output Voltage
8
2.1
B-device (SRP capable) to Host
Output Voltage
B-device (On-The-Go)
0V≤VBUS≤5.25V
Unconfigured Average Current
TAVG=1 ms
B-device (SRP capable, peripheralonly) Unconfigured Average Current
0V≤VBUS≤5.25V
TAVG=1 ms
OTG Device Leakage
Resistance to Ground on Mini-B Plug
Resistance to Ground on Mini-A Plug
mA
5.25
V
2.0
V
150
µA
8
mA
0.342
V
100
kΩ
10
Ω
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4.
USB in SAM9x5 Series and SAMA5D3x Products
4.1
USB Selection
For SAM9x5 series and SAMA5D3x products, there are possible use cases as below:
z
Three Hosts:
z
Two HS Hosts and one FS Host (SAM9x5 series)
z
Three HS Hosts (SAMA5D3x)
z
Two Hosts and one Device:
z
One HS Host, one FS Host (SAM9x5 series)
z
Two FS Hosts (SAMA5D3x)
z
One HS Device
Figure 4-1. USB Selection
FS (sam9x5)
HS (sama5d3)
Transceiver
HS
Transceiver
HS
Transceiver
EN_UDPHS
0
PC
PB
PA
HS USB Host
HS EHCI
FS OHCI
DMA
4.2
HS
USB
Device
DMA
USB Characteristics
z
z
4.3
1
Compliant with Enhanced HCI Rev 1.0 Specification
z
Compliant with USB V2.0 High-speed
z
Supports High-speed 480 Mbps
Compliant with Open HCI Rev 1.0 Specification
z
Compliant with USB V2.0 Full-speed and Low-speed Specification
z
Supports both Low-speed 1.5 Mbps and Full-speed 12 Mbps USB devices
z
Root Hub Integrated with X Downstream USB Ports
z
Shared Embedded USB Transceivers
z
Supports Power Management
z
2 Hosts (A and B) High Speed (EHCI) and Full Speed (OHCI), 1 Host (C) Full Speed only (OHCI) (SAM9x5 series)
z
3 Hosts (A, B and C) High Speed (EHCI) and Full Speed (OHCI) (SAMA5D3x)
USB Dedicated Function and DEMUX
Both Port C and Port B have dedicated functions. For SAM9x5 series, Port C supports Full speed OHCI (Open Host
Controller Interface) and Port B supports HS USB Host. For SAMA5D3x, Port C and Port B both support HS USB Host.
HS USB Host port A and HS USB Device share one transceiver. It is controlled by EN_UDPHS bit located in the USB
Device user interface.
Set to 0Æ Port A USB Host selected
Set to 1Æ USB Device selected
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5.
USB OTG Like Connector Implementation
5.1
Manual Operation
HHSDPA and HHSDMA bonding out pads are connected with receptacle, such as Mini-A receptacle. One receptacle can
be configured to host or device according to the customer requirements, just like USB OTG.
HHSDPA
HHSDMA
Figure 5-1. USB Selection for USB OTG Like Connector Implementation
HS
Transceiver
EN_UDPHS
0
1
PA
z
HS USB
Host
HS EHCI
FS OHCI
HS
USB
Device
DMA
DMA
Configured to Host
When USB gadget is not enabled in the code, set EN_UDPHS bit to 0 (this register is located in the UDPHS_CTRL
control register). The USB host is selected and programmed; use the port as host, which is able to detect USB devices
supported by the system, such as U-disk, Key board, Mouse, etc. (when connected to USB host, no functionalities).
z
Configured to Device
When USB gadget is enabled in the code, set EN_UDPHS bit to 1 (this register is located in the UDPHS_CTRL control
register). The USB device is selected and programmed; use the port as slave. When connected to USB host, it runs the
predefined functionalities (when connected to USB slave device, no functionalities).
5.2
Auto Detection
Two GPIOs are used: GPIO1 is for ID status detection and GPIO2 is for VBUS output control.
By default, the USB is configured to device and the GPIO2 disables the EK board as the USB VBUS power supply, waiting
for the external USB host power supply. When an external USB peripheral plug is detected, check the GPIO1 ID status.
If GPIO1=1, keep the same configuration. After getting the power supply from external host of VBUS, the device pulls up
the bus line and handshakes with the host.
When GPIO1 detects ID status, if GPIO1=0, which means ID Pin is connected to ground, the GPIO generates an
interrupt and then the CPU configures the USB as the host. Then GPIO2 enables VBUS to power supply the external USB
peripheral. The host pulls down the bus line, waits for the bus line pull-up signal from external USB peripheral, and then
handshakes with the external peripheral.
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5.3
USB OTG Like Connector Implementation Example Based on SAM9x5-EK
The implementation takes SAM9x5 series as an example. The case is similar for SAMA5D3x.
5.3.1
Hardware Implementation
Make some modifications on SAM9x5-EK board to realize the hardware implementation. Use two GPIOs (defined by
users) for ID status detection and VBUS output control respectively.
SAM9X35-EK is used in this example. On SAM9X35-EK board, add a sky-wire from Pin 4 (ID pin) of USB-A receptacle
(J20) to PA8 as GPIO1 for ID status detection, and use PD18 as GPIO2 for VBUS output control.
The schematic is shown as below:
Figure 5-2. Schematic for USB OTG Like Connector Implementation
3V3
R137
47k
L14
MN15
{14}
1
5V_INTER
2
8
5V
+ C106 220ohm at 100MHz
C108
33u
100n
L15
C107
100n
2
1
6
5
+ C110 220ohm at 100MHz
33u
C109
100n
7
OUTA
ENA
IN
FLGA
GNG
FLGB
OUTB
ENB
1
ACTIVE LOW
LCD_DETECT# {14}
2
3
PB17
OVCUR_USB
4
EN5V_HDA#
AIC1526-0GS
PD18
3V3
R138
C111
15p
J20
7
SHD
EARTH_USB
82k
VBUS
DM
DP
ID
GND
(VBUS_SENSE)
R139
47k
PB16
{3}
R140
47k
1
2
3
4
5
USBA_DM {3}
USBA_DP {3}
(IDUSBA)
6
PA8
G3515-09010101-00
EARTH_USB
5.3.2
Software Implementation
Make some modifications on the released Linux kernel source code for SAM9x5-EK to realize the software
implementation.
5.3.2.1 Software Environment
z
Linux OS
z
SAM-BA 2.11 or later version
z
Serial terminal emulator (HyperTerminal, minicom, Tera Term, etc)
5.3.2.2 Software Implementation
1.
Get the Linux kernel source code
2.
Modify the Linux kernel source code
z
Automatically modify the Linux kernel source code by applying a patch
A USB OTG like connector patch targeted at this example is provided. Find the
sam9x5ek_usb_otg_like_connector.patch in the attachment of this app note.
<get> sam9x5ek_usb_otg_like_connector.patch
Note:
Copy this file into linux-2.6.39 directory.
patch -p1 < sam9x5ek_usb_otg_like_connector.patch
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z
Manually modify the Linux kernel source code
For details on the modification of Linux kernel source code, please refer to “Example Codes for Linux Kernel
Modification” .
3.
Configure and build the new Linux kernel
For the detailed execution of the above steps, please refer to:
http://www.at91.com/linux4sam/bin/view/Linux4SAM/SAM9x5Page#Linux4SAM_AT91SAM9x5_Experimenta
5.3.2.3 Example Codes for Linux Kernel Modification
The code modification mainly occurs in three files: atmel_usba_udc.c, atmel_usba_udc.h, ohci-at91.c.
z
Configure USB to Host/Device by identifying the status change of ID pin and VBUS pin
Location: drivers/usb/gadget/atmel_usba_udc.h
/* Add the definition of udc->id_prev as the previous ID pin status for
Host/Device status change identification */
struct usba_udc {
...
u16 test_mode;
int vbus_prev;
int id_prev;
...
}
Location: drivers/usb/gadget/atmel_usba_udc.c
Target Function: static irqreturn_t usba_vbus_irq(int irq, void *devid)
/* If ID pin is pulled down, configure USB to Host */
if (!gpio_get_value(AT91_PIN_PA8)) {
udc->id_prev = 0;
usba_writel(udc, CTRL, USBA_DISABLE_MASK);
return IRQ_HANDLED;
}
if (udc->id_prev != gpio_get_value(AT91_PIN_PA8)) {
udc->id_prev = 1;
return IRQ_HANDLED;
}
Location: drivers/usb/gadget/atmel_usba_udc.c
Target Function: int usb_gadget_probe_driver(struct usb_gadget_driver *driver,
int (*bind)(struct usb_gadget *))
/* Only when VBUS is powered on and ID pin is float, configure USB to
Device */
if (vbus_is_present(udc) && udc->vbus_prev == 0 &&
gpio_get_value(AT91_PIN_PA8)) {
toggle_bias(1);
usba_writel(udc, CTRL, USBA_ENABLE_MASK);
usba_writel(udc, INT_ENB, USBA_END_OF_RESET);
}
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z
Control VBUS power for USB Host/Device
Location: drivers/usb/host/ohci-at91.c
Target Function: static irqreturn_t ohci_at91_otg_irq(int irq, void *data)
/* debounce */
mdelay(10);
if (gpio_get_value(AT91_PIN_PA8)) {
/* If ID pin is float, power off VBUS */
at91_set_gpio_output(AT91_PIN_PD18, 1);
} else {
/* If ID pin is pulled down, power on VBUS */
at91_set_gpio_output(AT91_PIN_PD18, 0);
}
z
return IRQ_HANDLED;
}
Register an interrupt for ID pin status
Location: drivers/usb/host/ohci-at91.c
Target Function: static int ohci_hcd_at91_drv_probe(struct platform_device *pdev)
/* Register an interrupt to identify the ID pin status */
int ret;
ret = request_irq(AT91_PIN_PA8, ohci_at91_otg_irq, 0, "otg_irq", pdev);
if (ret) {
dev_err(&pdev->dev, "request irq failed\n");
}
if (!gpio_get_value(AT91_PIN_PA8))
at91_set_gpio_output(AT91_PIN_PD18, 0);
5.3.3
USB OTG Like Connector Verification
After the modified Hardware and Software are both ready for use, verify the USB OTG like connector.
1.
Download the modified Linux Binaries into SAM9X35-EK board
2.
Open a Terminal for message printout
3.
Verify the USB OTG like connector
z
Connect a Host (PC, etc.) with the micro USB receptacle on EK board:
The EK board will be detected on the Host as a device.
z
Connect a Device (U Disk, portable devices, etc) with the micro USB receptacle on EK board:
The corresponding information will be displayed on the Terminal when plugging/unplugging the Device.
The USB OTG like connector is successfully implemented on SAM9X35-EK board.
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6.
Differences from USB OTG
On Hardware side, the Mini receptacle can be configured to host or device, just like USB OTG, and with the auto detect
solution, two GPIOs can be configured to support the features. GPIO1 detects ID pin status and automatically generates
interrupt of the status; GPIO2 handles VBUS output control.
However, the solution is still not the same with the real USB OTG that doesn’t need external detection circuit. The
differences between them are as below:
z
z
z
VBUS output voltage range:
z
USB 2.0: 4.75V to 5.25V
z
USB OTG: 4.4V to 5.25V
VBUS output current range:
z
USB 2.0: 100 mA to 500 mA
z
USB OTG: from 8 mA
HNP, SRP are not supported on USB in SAM9x5 series and SAMA5D3x products.
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7.
Revision History
Doc. Rev.
Comments
11201A
First issue.
Change Request
Ref.
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