HSIC vs USB: Key Differences Explained

This article explores the distinctions between HSIC (High-Speed Inter-Chip) and USB (Universal Serial Bus) interfaces. While both facilitate data transfer, HSIC is gaining traction for hardwired, inter-chip communication due to its specific advantages over standard USB.

HSIC provides USB high-speed data transfer rates of up to 480 Mbps using a two-signal, source-synchronous interface. It’s designed to be 100% compatible with existing USB host drivers.

It’s important to note that Full Speed (FS) and Low Speed (LS) modes aren’t natively supported. However, a hub with HSIC support can bridge the gap and enable FS and LS compatibility. The primary difference between HSIC and traditional USB lies in the physical layer implementation.

Key characteristics of HSIC include the absence of a chirp protocol and the utilization of source-synchronous serial data communication. Being a permanently connected interface, it doesn’t support hot removal. All data transmission in HSIC occurs over a single data line, with a strobe signal indicating when the received data should be sampled. HSIC uses double data rate (DDR) signaling, sampling data on both the rising and falling edges of the strobe signal. The strobe signal oscillates at 240MHz, resulting in an aggregate data rate of 480Mbps. The communication protocol between the host and device remains consistent with the standard USB interface.

The following table provides a detailed comparison of HSIC and USB:

HSIC vs USB: A Comparison Table

Feature	USB	HSIC
Full form	Universal Serial Bus	High-Speed Inter-Chip
Data rate	Different versions support varying data rates: USB V1.x (up to 12 Mbps), USB 2.0 (about 480 Mbps), USB 3.0 (5Gbps), USB 4.0 (10Gbps)	480Mbps
Maximum cable length	5 meters (@ 480 Mbps)	5 meters (@ 480 Mbps)
Size	The presence of an analog front-end contributes to a larger size.	The absence of an analog front-end results in a smaller size.
Cost	More expensive compared to HSIC devices.	Cheaper compared to standard USB devices.
Interface	Analog.	Digital.
Interface lines	D+, D-	Strobe, Data
Current consumption	Draws 200 µA on D+ through 1.5 KOhm (in suspended mode)	Draws no current on strobe/data in suspended mode
Power consumption	Higher compared to HSIC-based USB versions	Draws 50% less compared to standard USB 2.0
Maximum allowed skew	Approximately 100 ps in a cable.	HSIC electrical spec defines about 15 ps.
Voltage signal level	5V (Vcc) and 3.3V (High signal level). Differential “1” is transmitted by pulling D+ over 2.8V with 15KOhm pulled to GND and D- under 0.3V with 1.5KOhm pulled to 3.6V. In differential “0,” D- is greater than 2.8V, and D+ is less than 0.3V.	Uses 1.2 V (LVCMOS levels) instead of 3.3 V for low-power applications
Probing method	Standard USB is easily monitored by placing a differential probe connected with an oscilloscope.	HSIC signals are very sensitive, and general transmission line theory is applied to probe the signal. Generally, probe at the opposite side of the signal source that needs to be monitored. To observe the signal originating from the “device”, probe on the “host” side and vice versa.
Data Sampling/Synchronization	Standard USB uses a sync pattern with every data packet to allow the receiver clock to synchronize with the incoming data’s phase.	HSIC uses a strobe line to tell the receiver when to sample the incoming data.