USB HSIC interface

You have seen USB2.0/USB3.0 interfaces which are used for connecting processor to external peripherals. The USB lines extend to a connector from which you connect a key-board, mouse, printer or whatever device you want. There are these other interfaces like I2C, SPI which we use for chip-to-chip connectivity. The only disadvantage of these interfaces is speed. Compare the USB2.0 rate of 480 Mbps with I2C speed up to 3.4 MHz which is very less. So, if we want to achieve such high speeds on a board for chip-to-chip connectivity there is a need for new interface which is USB HSIC which was introduced in 2007. So, this is quite new to many hardware engineers. Design engineers working on latest high end processors, SoCs must be familiar with these interfaces.

USB HSIC is basically a synchronous serial interface used for chip-to-chip connectivity. It is basically a two signal interface that uses  1.2 V LVCMOS signalling for communication. The main difference here with the USB is that USB uses a 3.3V signalling. Another difference is that USB uses analog interface externally where as HSIC uses digital signalling on board. As HSIC eliminates analog transceivers, it is low cost, low power consuming and low complexity interface. Another advantage is the low board space required for signals.  As it is a chip-to-chip connectivity interface, there is no need for external cables or connectors.It does not support plug-n-play. also, hot plug not supported.

USB HSIC is a two signal interface which uses DATA, STROBE signals for communication. USB HSIC strobe signal uses 240 MHz clock. It uses dual data rate to transmit data at 480 Mbps. A dual data rate interface is one which clocks data over both leading and falling edges of the clock. One thing to remember here is data and strobe signals are bi-directional. It uses NRZI encoding on signals.

USB HSIC uses the same software stack as USB and can be said as high speed connectivity interface between chips. The main disadvantage is that latency of USB HSIC is little high. HSIC is used mainly in battery powered applications because of low power consumption. USB HSIC can be expanded to connect external USB interfaces using HSIC hubs like USB2513, USB3503, USB4640.

Applications:

• Smartphones
• Desktops
• Tablets
• set-up boxes
• GPS navigators

USB3.0 also has a version of chip-to-chip connectivity which is called SSIC. 

Probing USB HSIC signals:

• We already have a set ups in place for USB measurements. The same cannot be used for USB HSIC as the PHY structure is different from USB. 
• High Frequency passive probes cannot be used and you may need a active probes. Only problem is these active probes are too costly to buy.
• For probing a USB HSIC signal at 480 Mbps you need a probe of band-width minimum 1.5 GHz.
• If we probe HSIC with a passive probe, there is always a chance that the interface may get reset.
• As per USB HSIC standard, the load cannot a capacitance of more than 14 pF. So, while choosing a probe keep this in consideration.
• The following table indicates the state of interface as per the signal states of data, strobe:

Design Engineer considerations:

• PCB trace length of HSIC signals should not exceed 10 cm (< 4 inches).
• Signals must be routed with a 50 ohm impedance
• Trace length matching must happen so that the skew will not exceed above 15 ps.
• Trace spacing should be 3 times the dielectric thickness

Other salient features of USB HSIC:

• Power consumed is very less when in idle state
• HSIC uses the same tiered topology as like USB2.0, the peripheral connectivity as shown in the below figure:

• There is no speed detection mechanism in HSIC. It defaults to USB2.0 speed.
• Data is transferred from USB HSIC only after idle condition, then data gets stopped after idle condition is back.

Tools for HSIC probing:

• Agilent Technologies have a HSIC compliance software which can do the following functions:
• Rise and Fall time measurements
• Bus state and timing measurements
• Packet parameters
• Data eye and mask testing
• test report generation
• Lecroy also has HSIC decode application for their scopes