Electronic Design

USB On Board: Flash Memory Leads The Way

The Universal Serial Bus provides needed flexibility and bandwidth for built-in high-speed peripherals.

A Universal Serial Bus (USB) interface is the norm for mid- to high-end microcontrollers and embedded systems, yet it doesn't have to be used just for external devices. It equally suits internal devices, but a more robust, board-level connector has been missing from the picture.

M-Systems' USB 2.0-based µDiskOnChip (µDOC) flash memory module comes at the right time. High-speed serial connections like PCI Express and Serial ATA bring significantly higher performance. Still, this tends to be overkill for the types of devices that USB supports. In fact, both of those interfaces are designed to complement, not replace, USB.

The µDOC will be successful regardless of third-party support of the USB interface, although this type of expansion is just the thing many embedded designers are seeking. The µDOC provides a low-cost, high-reliability system that's more flexible than M-Systems' successful DiskOnChip product. Why? Simple. A USB interface provides access to any number of devices, whereas a dedicated DiskOnChip socket is just that—dedicated.

Another advantage of the µDOC is that it can be used with any operating system that supports USB disk devices. It can even be used as a boot device. USB devices of this type are interchangeable, supporting alternative peripheral configurations.

The µDOC is essentially a repackaging of M-Systems' popular DiskOnKey technology. The DiskOnKey is a flash memory dongle that plugs into a USB port. It differs from many alternative products, as it uses an ARM processor and has built-in security features. These features include incorporation of keys and encryption technology right on the flash memory chip for a high degree of security. The processor also enables the system to transparently handle sector leveling.

The µDOC module is built around an 18-pin header that provides a solid connection to a USB device. At this point, the µDOC is the only product available for this form factor, yet it's applicable to any USB peripheral.

The µDOC comes in vertical and horizontal form factors. The horizontal alignment allows the µDOC to be bolted to the board, preventing accidental removal. The vertical form factor shown in the figure allows easy insertion and removal.

Many boards use an eight-pin header with dual four-pin USB ports, but this new design provides a more solid base. The additional pins improve power distribution and expansion for future options.

The µDOC we tested came with a small adapter that consisted of a USB connector and an 18-pin header. The adapter plugged into a standard USB interface, letting us program the unit via a PC. The µDOC was then removed from the adapter and plugged into a system with USB ports where we had connected an 18-pin header to the on-board USB support. It is a very convenient way to update an operating system or application. The µDOC uses the same drivers as the DiskOnKey that we had used on the PC.

Performance is impressive with a read throughput of 9 Mbytes/s. Write performance is slightly slower but very suitable for most storage requirements. Pricing is under 30 cents per megabyte with capacities up to 1 Gbyte available now.


Feature Description
Interface USB 2.0
Capacity 128 Mbyte to 1 Gbyte
Encryption DES (56-bit key), SHA-1, RSA
3DES (112-bit key or 168-bit key)
Package 18-pin (9 by 2) header
48 by 30 by 5.9 mm
Vertical or horizontal alignment
Performance 9-Mbyte/s read
7-Mbyte/s write
Reliability TrueFFS flash management
On-the-fly EDC/ECC up to 4 errors/page
Power 95 mA, 3.3 V or 5 V
Operating temperature 0°C to 70°C

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.