Micro/sys is the original developer of the StackableUSB standard, so I decided to pick their brains to find out more on this new architecture.
| Electronic Design: | Is the StackableUSB a standard? What type of licensing terms are available? |
| Micro/sys: | The specification defines the standard, and therefore StackableUSB is a standard. It is available for licensing. Details are provided at stackableusb.org. |
| ED: | USB has been found on SBCs for many years. How did the demand for more USB peripherals in embedded systems drive the design of StackableUSB? |
| Micro/sys: | The demand for USB itself was not the motivator for StackableUSB. The demand was actually for a replacement for ISA that is of similar performance and cost. ISA is becoming more difficult and costly to include in SBC designs as it approaches obsolescence. Other faster, more expensive, and complex busses have failed to displace ISA. We feel that USB is the best choice to replace ISA. |
| ED: | StackableUSB uses a new connector. Why was it designed, and what features does it bring to the embedded USB environment? |
| Micro/sys: | The Q2 connector is rugged, limits crosstalk between differential pairs, and supports high frequency signaling. |
| ED: | How important is StackableUSB's bolt-down orientation in an embedded environment? |
| Micro/sys: | This is addressed in the StackableUSB FAQs. |
| ED: | StackableUSB uses differential signals. What advantages does this approach have, and what impact does it have on system design? |
| Micro/sys: | USB uses a single differential pair per point-to-point connection between the USB host and each USB device. Therefore USB dictates the use of differential signaling. Differential signaling is intended to reduce the impact of common mode noise on digital signals. |
| ED: | Will there or could there be a version of StackableUSB that uses single-ended signals? What might the advantages or disadvantages be? |
| Micro/sys: | No. USB does not support single-ended signaling. |
| ED: | The idea for a stackable USB architecture has been around for a while, but PCI Express beat it out the door with the stackable EPIC Express standard. Why did StackableUSB take longer to come to fruition? |
| Micro/sys: | Good discernment is an ongoing process, and market opportunities change as time passes. USB had been out for a while when the more complex, high-bandwidth PCI Express emerged. As EPIC Express implemented PCI Express for embedded systems, the concern with USB was that it would be phased out of CPUs and chip sets, just as ISA was quickly being eliminated. But USB kept getting stronger with USB 2.0, USB On-the-Go, and Wireless USB. More significantly, USB moved beyond the PC world to other CPU cores such as microcontrollers and ARM XScale processors. This expanding presence of USB could not be ignored. |
| ED: | Seven adapter boards seems like more than enough expansion, given that most PC/104-style stacks have an average of 2.5 boards. Still, would it be possible for SBC designers to provide another stack on the bottom of an SBC for further expansion? |
| Micro/sys: | The StackableUSBTM specification does allow for support for up to 10 peripherals from a single SBC without the use of hub cards on top and bottom. |
| ED: | How well does StackableUSB mix with EPIC Express? Would this combination allow full support of the ExpressCard standard that mixes USB and PCI Express x1? |
| Micro/sys: | StackableUSB in its 15.24-mm board-to-board spacing form is fully compatible with Epic Express. In other words, it is possible to design an SBC that is capable of controlling both StackableUSB peripherals and Epic Express peripherals simultaneously. It is further possible to design a single peripheral that supports both interfaces. |
| ED: | The possibility of smaller StackableUSB card form factors has been mentioned. What is the status of these, and what kinds of options might be available? |
| Micro/sys: | The most recent revision to the StackableUSB specification specifies the ½-size peripheral card and also allows up to two ½-size stacks on both the top and bottom of the SBC. We envision the emergence of smaller formats. |
| ED: | Could the StackableUSB architecture be implemented using even smaller connectors for very small embedded systems? |
| Micro/sys: | In theory, yes, and the beauty of StackableUSB is the growth that is possible in the future. That being said, planning and care need to be taken to protect the integrity of the StackableUSB standard for the user. |
Reprints
