2020 Microchip Technology Inc. DS00003535A-page 1 INTRODUCTION The Microchip USB24926C and USB24926P smart hubs allow for smartphone-automotive mode sessions to be entered on the downstream ports via a Dual Upstream (DUST) mode of operation. • The USB24926C supports the Dual Upstream operation on four downstream ports, where two ports support basic USB Type-C ® . • The USB24926P supports the Dual Upstream operation on four downstream ports, where two ports support USB Type-C with Power Delivery. The USB24926C and USB24926P hubs have two upstream connections. The primary upstream connection is used for general hub operation and is the only connection utilized when there is no active smartphone-automotive mode session. When an automotive session is initiated for a downstream port, that port is switched over to the secondary upstream port. The secondary upstream port is connected to a USB device or OTG port that operates in Device mode during the smartphone-automotive mode session. This allows the smartphone to act as the USB host via the secondary upstream connection, while all other USB ports continue to operate as standard downstream ports with a connection maintained to the USB host on the primary upstream connection. A DUST session may be entered on only one downstream port at a time. Entry into DUST mode is initiated via USB Control transfer addressed to the internal Hub Feature Controller (HFC) device in the hub. Sections This document includes the following topics: Functional Overview on page 2 DUST Command Details on page 6 Dual Upstream Mode Command Example on page 8 Additional Operation Options on page 9 References Consult the following documents for details on the specific parts referred to in this document. • Microchip USB24926C Data Sheet • Microchip USB24926P Data Sheet • Microchip Configuration Options for USB249XXX Application Note AN3535 USB24926C/USB24926P Dual Upstream Operation Author: Josh Averyt Microchip Technology Inc.
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2020 Microchip Technology Inc. DS00003535A-page 1
INTRODUCTIONThe Microchip USB24926C and USB24926P smart hubs allow for smartphone-automotive mode sessions to be enteredon the downstream ports via a Dual Upstream (DUST) mode of operation.• The USB24926C supports the Dual Upstream operation on four downstream ports, where two ports support basic
USB Type-C®.• The USB24926P supports the Dual Upstream operation on four downstream ports, where two ports support USB
Type-C with Power Delivery.The USB24926C and USB24926P hubs have two upstream connections. The primary upstream connection is used forgeneral hub operation and is the only connection utilized when there is no active smartphone-automotive mode session.When an automotive session is initiated for a downstream port, that port is switched over to the secondary upstreamport. The secondary upstream port is connected to a USB device or OTG port that operates in Device mode during thesmartphone-automotive mode session. This allows the smartphone to act as the USB host via the secondary upstreamconnection, while all other USB ports continue to operate as standard downstream ports with a connection maintainedto the USB host on the primary upstream connection.A DUST session may be entered on only one downstream port at a time. Entry into DUST mode is initiated via USBControl transfer addressed to the internal Hub Feature Controller (HFC) device in the hub.
SectionsThis document includes the following topics: Functional Overview on page 2 DUST Command Details on page 6 Dual Upstream Mode Command Example on page 8 Additional Operation Options on page 9
ReferencesConsult the following documents for details on the specific parts referred to in this document.• Microchip USB24926C Data Sheet• Microchip USB24926P Data Sheet• Microchip Configuration Options for USB249XXX Application Note
AN3535USB24926C/USB24926P Dual Upstream Operation
Author: Josh AverytMicrochip Technology Inc.
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FUNCTIONAL OVERVIEWThe internal block diagram of Microchip USB24926C and USB24926P is shown in Figure 1.Both USB24926C and USB24926P have an internal USB device called the Hub Feature Controller, which enables theadvance features of the hub. The Hub Feature Controller device is a standard USB device class (0xFF Vendor Specific)with Product ID as in Table 1.• On the USB24926C and USB24926P, the Hub Feature Controller is connected to port 5.The hub port that is connected to the Hub Feature Controller is configured as non-removable.
Dual Upstream Automotive SessionA DUST session is initiated via a USB command to the internal Hub Feature Controller device. The details of this USBcommand are shown in DUST Command Details on page 6, and an example USB protocol trace capture of the com-mand is detailed in Dual Upstream Mode Command Example on page 8.The USB24926P supports DUST operation on downstream ports one and two. The USB24926C and USB24926P sup-port DUST operation on each of the four downstream ports.When a DUST session is initiated on one of the downstream ports, the port is multiplexed over to a secondary one-porthub device. This creates a dedicated connection between the smartphone operating in automotive Host mode and theautomotive head unit.
AN3535A typical sequence of events for automotive session initialization is:1. The automotive head unit (USB Host) enumerates a smartphone on DUST capable port “X” and discovers that
the smartphone is capable of an automotive media session.2. The automotive head unit sends any necessary commands to the smartphone to set that device into the auto-
motive mode.3. The automotive head unit sends the DUST command to the internal Hub Feature Controller device Endpoint 0
to switch port “X” into DUST mode.4. The automotive head unit detects that the smartphone is disconnecting from port “X”.5. The smartphone switches to Host mode operation and detects a one-port hub attach.6. The smartphone detects the automotive head unit (via its separate USB Device mode connection) is connected
on the USB tier below the one-port hub.7. Data can now be exchanged between the smartphone operating in USB Host mode and the automotive head
unit’s USB Device mode port.Figure 2 shows the internal connections made both before and after an automotive session is initiated.
FIGURE 2: DUAL UPSTREAM OPERATION
Device Mode
Host Mode
Before Automotive Session Initiation
After Automotive Session Initiation
USB Host
Upstream Port
USB24926x Hub Controller Logic
DUST Port
One-Port Hub
OTG/Device Port
Secondary Downstream
Port
UpDown
Up
Down
USB Host
Upstream Port
USB24926x Hub Controller Logic
DUST Port
One-Port Hub
OTG/Device Port
UpDown
Up
Down
Secondary Downstream
Port
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Forced Session TerminationIf at any time the USB Host (automotive head unit) intends to forcefully terminate the automotive session, the recom-mended method is to issue the DUST Command to the HFC (see DUST Command Details on page 6) with the com-mand parameters set as follows:• ENUM_TIMEOUT = 000b• ROLE_SWITCH_TYPE = 10b• ROLE_SWITCH_STATE = 0b• PORT = 0000bUpon receipt of this command, the hub terminates the automotive session by detaching the secondary one-port hub.Thesmartphone will recognize that the entire hub and device tree detach from the bus, and the smartphone reconfiguresitself to revert to its default operation as a USB device. Finally, the smartphone will re-enumerate to the USB host (auto-motive head unit) as a USB device.
Automotive Session Launch FailureIf there is an error in the process of launching of an automotive session, then a mechanism for detecting launch failureand a recovery mechanism are required. A launch failure may occur for any of the following example reasons:• The smartphone is rapidly disconnected before the secondary one-port hub is fully enumerated (for example, prior
to the ‘Set Configuration’ command being sent) by the smartphone.• An event occurs on the downstream port’s VBUS line that results in a momentary loss in power to the smartphone
before the secondary one-port hub is fully enumerated (the ‘Set Configuration’ command is sent) by the smart-phone. A typical smartphone behavior is to revert to USB Device mode if a momentary loss of power is detected while in an automotive session.
• There is an error or bug in the smartphone OS/software.• Cable or connector is mechanically loose.• USB signal quality is poor resulting in communication failure. Common reasons include bad cable (poor wiring or
USB shielding or insufficient VBUS/GND current capacity), bad connection, wireless interference, or poor quality PCB design.
In the event that an automotive session launch failure does occur, an appropriate “time-out” recovery method must beimplemented, based upon the automotive media interface specification’s guidance.As an example, if smartphone authentication (where applicable), the NCM network is successfully established, and thedata flow between the USB host (automotive head unit) and the smartphone has initiated successfully, then the headunit system can assume a successful session launch.However, if the NCM network fails to establish within 1-2 seconds after attempting to launch, the head unit system canconclude that a failure occurred, and reverts the hub to the default state (by resetting the hub, or issuing the DUST com-mand as shown in Forced Session Termination on page 4) in order to try again.
Disconnection DetectionIf for any reason the smartphone is detached during the automotive session after the secondary one-port hub has com-pleted enumeration with the smartphone, the USB host (automotive head unit) must be able to detect that the detachhas occurred. The disconnection detection mechanism can be one of the following:• (Recommended if ENABLE_AUTO_ROLE_REVERT = 1) The smartphone reappears in the USB device tree as
a device. Additionally, the data flow between the head unit’s Device mode port and the smartphone stops. The head unit’s Device mode port may observe this condition as a USB SUSPEND bus state.- The USB hub uses one of two methods for detecting when the smartphone has been physically detached
from its port. This mechanism is further explained in Disconnect Detection Method on page 9.• (Recommended if ENABLE_AUTO_ROLE_REVERT= 0) The data flow between the head unit’s Device mode
port and the smartphone has stopped. The head unit’s Device mode port may observe this condition as a USB SUSPEND bus state.- To restore the hub system to its default state in this case, the USB host must reset the hub or issue the DUST
command as shown in Forced Session Termination on page 4.- In typical automotive sessions, a video stream is constantly being transmitted from the smartphone to the
automotive head unit. When the smartphone is detached in this case, that video stream data from the smart-phone will no longer be received by the automotive head unit’s Device mode port.
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AN3535Hub Port Disabling and RemappingThe USB24926C and USB24926P have flexible port configuration options allowing for ports to be disabled via SMBus,OTP, or pin strap. Alternatively, by using the PortMap feature, individual hub port numbers can be logically remappedor disabled so that the port ordering seen by the USB Host can be customized. The mapping table for PortMap can beconfigured during the SOC configuration process with SMBus or by programming the OTP. Only one of these configu-ration methods should be used to configure the ports; using both configurations methods in parallel may result in anunexpected port configuration.If either of these configuration options is implemented, it may affect the parameters sent by the USB Host when usingthe DUST role swap feature. The DUST command contains four PORT bits that select the target port. The target portdefined by these bits is always linked to the physical pins associated with that port. If the target port has been logicallyremapped, its enumerated port number may not match the physical port number needed for the PORT argument in therole swap command.Example: Port 2 is disabled via pin strapping, and ports 3 and 4 are logically remapped to ports 2 and 3, respectively.Ports 2 and 3 are automatically remapped by the hub since discontinuities in port numbering is not possible due to theway the port numbering is assigned per the USB specification. To enter a DUST session with Logical Port 2 (PhysicalPort 3), the DUST command must be issued to the hub with target port 3 selected (PORT = 0011b).The following guidelines are recommended for applications that will use the DUST function and custom port configura-tion together:• When disabling ports, start with the highest number port to ensure that no DUST-capable ports are automatically
remapped.• Avoid using the port remap feature.• If port remapping cannot be avoided, it is important to use only one method of configuration, whether it be pin
strapping, SMBus configuration, or OTP configuration. Using multiple configuration methods increases the risk of configuration collisions from occurring and the risk of encountering unexpected results.
While the DUST feature can still be successfully used when ports are automatically or manually remapped, additionalcare must be taken to ensure that the target port is correctly selected when issuing the DUST command. There are twostrategies that can be employed if port remapping is unavoidable:• Program or configure a product-specific serial string or product string that the host software can use. The host
software can implement a capabilities look-up table to correctly map the physical port with the logical port.• The USB host may interrogate the hub’s port disable registers (if using port disable strapping) and/or the port
remap register (if remapping ports manually) to discover the logical mapping with respect to the physical mapping.
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DUST COMMAND DETAILSAll commands related to the DUST feature are sent to the USB HFC within USB24926C and USB24926P hubs.Depending on the application configuration of USB24926C/USB24926P, certain other USB features may be enabled ordisabled, which alter the USB endpoint configuration of the HFC that has the effect of altering its Product ID value. TheHFC will exhibit one of the following Product IDs (see Table 1), according to these configuration options:
The USB hub requires a specific USB command to be issued from the USB host to indicate when a DUST automotivesession should begin.The USB command is a NO DATA Control transfer that must be issued to Endpoint 0 of the internal Hub Feature Con-troller device. On USB24926C and USB24926P, the Hub Feature Controller is the internal device located on Port 5.The SETUP command format is shown in Table 2 and Table 3.
Product IDNo Yes No No Only I2S audio interface —No Yes Yes No I2S audio and HID interfaces —Yes No No No HFC only, PID maintained out of sequence for
MPW compatibility0x49A0
Yes Yes No No HFC and I2S audio interface 0x49AA
Yes Yes Yes No HFC, I2S audio and I2S HID interface 0x49AB
Yes No No Yes Only HFC and Type-C bridge 0x49E8
TABLE 2: DUAL UPSTREAM MODE SETUP PACKETSetup Parameter Value Description
bmRequestType 0x41 Device-to-host, vendor class, targeted to interfacebRequest 0x90 SET_ROLE_SWITCHwValue 0xYYYY Bits detailed belowwIndex 0x0000 ReservedwLength 00 No data
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TABLE 3: WVALUE DETAIL OF DUAL UPSTREAM MODE SETUP PACKETBit Name Description
15:11 Reserved Reserved (Must Always Be ‘0’)10:8 ENUM_TIMEOUT For DUST applications, 000 should also be selected.
000 = No Time-out defined (hub will not automatically terminate the DUST session)All others = Not Recommended
7 Reserved Reserved (Must Always Be ‘0’)6:5 ROLE_SWITCH_TYPE Must always be ‘10’ for DUST mode4 ROLE_SWITCH_STATE 1 = Transitions the port defined in PORT bits to DUST session
0 = Terminates the DUST session that is currently active on the port defined in PORT bits
3:0 PORT Logical downstream port to initiate or terminate DUST session
0001 = Port 10010 = Port 20011 = Port 30100 = Port 4All others = Invalid
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DUAL UPSTREAM MODE COMMAND EXAMPLEAn example of the DUST session initialization command for Port 3 is shown in Table 4 and Figure 3. This command issent to EP0 of the Hub Feature Controller.
AN3535ADDITIONAL OPERATION OPTIONSBy default, no additional configuration is required for DUST mode to operate. However, there are many additionaloptions available that may be useful for system integration.
Disable Auto-Role RevertingUnder certain specific conditions (while in a DUST session), the secondary one-port hub will automatically detach andthe primary hub will return to its default state (automatic role reversal). This may happen for one of two reasons:• An overcurrent event for the respective DUST mode port is detected.• The host attached to the secondary one-port hub is detected as disconnected (see Disconnect Detection Method).This functionality can be disabled by clearing bit 1 of register BFD2_3402h, the Runtime Flags 2 registers. See Table 5.
Disconnect Detection MethodThere are two different mechanisms for detecting when a host connected to the upstream port of the secondary one-port hub is physically detached:• SUSPEND TIMER: This is the default disconnect detection mechanism. When the secondary host is detached
from the secondary one-port hub, the hub detects a USB SUSPEND event (due to lack of SOF commands from the host). While in the SUSPEND state, a timer runs. When the timer expires, the secondary one-port hub con-cludes that it has been detached from the secondary host and initiates the role reversal to return to the default state. Note that this method is suitable for most DUST implementations as most DUST sessions remain active during the entire session. Hence, the secondary host will never place the secondary one-port hub into the SUS-PEND state. The SUSPEND timer is configurable via register BFD2_340[C:D]h. See Table 6.
• CURRENT INJECTION ON USB DATA LINES: When the secondary host is detached from the secondary one-port hub, the hub detects a USB SUSPEND event (due to lack of SOF commands from the host). While in the SUSPEND state, the secondary one-port hub connects a weak current source to the D- data line of the upstream port and then monitors the line state of the pin. If the pin line state is detected as low, then the secondary one-port hub concludes that it is still attached to the secondary host. If the pin line state is detected as high, then the sec-ondary one-port hub concludes that it has been physically detached from the secondary host and initiates the role reversal and returns to the default state. This mechanism may be useful for operating with secondary hosts that may place the secondary one-port hub into suspend while in an active DUST session.
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Secondary One-Port Hub Attach DelayA configurable amount of delay can be inserted between when the DUST command is issued, and when the secondaryone-port hub appears on the port.This is intended for debug or test use only and is not recommended for final implementations where the USB host shouldissue the command at the most appropriate time. This delay is programmed is register BFD9_51A8h. See Table 7.
Bit Name R/W Description31:17 Not Related R Settings Not Related to DUST operation (do not change)
16 RFL_ENABLE_AUTO_ROLE_REVERT
R/W 0 - While the ports are in role-switched mode, only explicit commands issued from the USB host can revert the roles to the default state.
1 - While the ports are in role-switched mode, a disconnection detected on the secondary one-port hub’s upstream port forces an automatic role revert to the default state.
15 RFL_DISCONNECT_DETECT_USBDATALINES
R/W This bit is ignored if: RFL_ENABLE_AUTO_ROLE_REVERT = 0
0 - The hub runs a timer when the upstream port of the one-port hub is placed into USB SUSPEND and will auto-detach when the timer expires. This timer duration is configured in the SUSPEND TIMEOUT register.
1 - The hub runs a check on the USB data lines to determine if the UDC2 has been physically detached. This is performed by injecting a small current source on the D- pin and checking the line state.
14:0 Not Related R Settings Not Related to DUST operation (do not change)
Bit Name R/W Description31:0 SUSPEND_TIMEOUT R/W This field can only take values that are integral multiples of 5.
By default, the setting is 0 ms.
This field refers to the minimum duration (in milliseconds) of USB SUSPEND that the upstream port of the one-port hub will stay within the SUSPENDED state before the one-port hub auto-detaches and reverts to the default state.
Refer to RFL_ENABLE_AUTO_ROLE_REVERT and RFL_DISCONECT_DETECT_USBDATALINES bits in RUNTIME_FLAGS2 for more details.
This field is ignored if RFL_DISCONNECT_DETECT_USBDATALINES = 1.
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TABLE 7: ROLE SWITCH DELAY REGISTER
ROLE_SWITCH_DELAY (DWORD)(BFD9_51A8h) Role Switch Delay
Bit Name R/W Description
31:0 ROLE_SWITCH_DELAY R/W This field refers to the amount (10-millisecond intervals) by which the role switch will be delayed once the DUST Com-mand is received.
For example, a register value of 2 provides 20 ms of delay before the role switch is initiated (one-port hub is switched).
TABLE 8: DUAL-UPSTREAM CONNECTION PORT STATUS REGISTER
DUAL-UPSTREAM STATUS (DWORD)(BF80_083Ch) Dual-Upstream Connection Port Status
Bit Name R/W Description
31:3 Not Related R Reserved (Always 0)
2:0 SEC_HUB_SEL R Indicates the downstream port that is connected to the Dual-Upstream (DUST) port.
000 - No Downstream port is connected to the DUST port.001 - Downstream port 1 is connected to the DUST port.010 - Downstream port 2 is connected to the DUST port.011 - Downstream port 3 is connected to the DUST port.100 - Downstream port 4 is connected to the DUST port.101 - 111 Invalid
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APPENDIX A: APPLICATION NOTE REVISION HISTORY
TABLE A-1: REVISION HISTORYRevision Level & Date Section/Figure/Entry Correction
DS00003535A (07-20-20)
Initial release
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AN3535NOTES:
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