HSIC Interface Description

High-Speed Inter-Chip USB [HSIC]; a USB chip-to-chip interconnect.
HSIC removes the analog transceivers found in normal USB. [Description of the USB Interface]
The maximum trace length defined is 10cm.
HSIC uses 2 signals (strobe, data) in a source synchronous serial interface with 240MHz DDR signaling to provide a 480Mbps USB interface.
The electrical interface uses 1.2 volt LVCMOS logic levels. Graph of Low Voltage Switching Levels, for both CMOS and TTL.
Both Data and strobe are bi-directional utilizing NRZI encoding. [Information Encoding]
There are five static bus conditions that are used to indicate operation;
ConditionStrobeDataIdleHiLoConnectLoHiResumeLoHiSuspendHiLoResetLoLo

The USB interface is well documented and widely used as an interface.
This versions allows board only designs while still keeping all the important USB features.
This interface is also Point-to-Point to match the normal USB interconnect.
Of course most other high-speed serial buses are also point to point [if for no other reason than to drive only one load].

There are a number of interface buses that only allow Chip-to-Chip connections, normally because they can't drive a larger load.
Other IC to IC Buses or buses interconnecting chip to chip.
There is no physical interface with these bus types, as the physical connection is always a copper trace between the ICs.
The switching levels or the electrical interface and the protocol, how they transmit the data, is the only thing that changes.