An Innovative Solution to an Old Problem

With nearly 20 man-years experience in the field of Embedded Microprocessors, we well remember the days when changing firmware involved erasing an EPROM for 20 minutes in a UV eraser, programming it in a PROM programmer, and finally plugging the EPROM into the target board. During development & debugging this would soon become very tedious! With the advent of electrically-erasable PROMs, the UV eraser could be dispensed with. By removing the need for a quartz window to admit UV light, the expensive ceramic package was no longer needed and cheap plastic encapsulation could be used instead. With Flash EPROMs and In-System Programming (ISP), today even the PROM programmer is largely redundant.

But even Flash does still have its limitations when developing & debugging embedded code:

• It is relatively slow.

  A vast improvement on the erase times of the old UV EPROMs, but still a bottleneck when frequent code changes are being made.

• It is read-only

  Well, of course - that's what "ROM" means, doesn't it?!

  The problem comes in debugging when software breakpoints are required. Normally, these are created by changing the opcode at the required breakpoint location to a special "breakpoint" instruction - but this is impractical with Flash, which does not allow individual bytes to be programmed at will.

The Triscend E5 offers an escape from these limitations:

The JTAG interface, together with the extreme flexibility of the E5's memory architecture, allows in-circuit code download & program execution from the internal SRAM, an external SRAM, or an external Flash device. In developing the new GSM STU, Versus Technology took advantage of these features by modifying some prototype boards to replace the normal Flash socket with an SRAM. Off-the-shelf pinout adaptors were used to mate the SOJ-packaged SRAM to the PLCC socket site (click the photo for a larger image).

While this adaptation greatly enhances development & debugging by removing the inconveniences noted above, there are still some limitations; eg:

• It is an invasive procedure!
  Thus, for example, it cannot be used if some investigations are required on a customer's unit.

• It is somewhat cumbersome and prone to damage.

• It is not reversible
  Therefore, when tests with a real flash device are required, a different unit is needed.

In response to these issues, Antronics Ltd conceived the idea of a custom Flash-replacement SRAM module that would simply plug into the standard Flash socket. Detailed design was completed by Versus Technology, and modules were produced by Winslow ADAPTICs.

These modules have proved a great success, and have been used in several subsequent development projects: