Flash - Master of the Universe

The 28C256 EEPROM, which holds the BIOS, has been our go-to choice since the early breadboard days. Before that, we relied on EPROMs, which required UV erasure before every upgrade—a time-consuming and inconvenient process. Switching to an EEPROM significantly improved our workflow, reducing turnaround time.

However, the 28C256 has a couple of drawbacks:

  • It cannot be written in-circuit since it requires a 12V programming voltage, which the Steckschwein does not provide.
  • It is slow, with an access time of 150ns, forcing us to introduce wait states to slow down the CPU when accessing the BIOS.

Because of these limitations, using ROM routines isn’t ideal from a performance standpoint. As a result, the BIOS’s sole function is to load the steckOS bootloader from an SD card into RAM and start it.

To overcome these issues, we designed an adapter board that replaces the 28C256 EEPROM with a Flash EEPROM, such as the 39F040, which features a much faster 55ns access time.

flash adapter

With this upgrade, the Steckschwein SBC now has 1MB of total memory—512K RAM and 512K Flash EEPROM—running at full speed. This unlocks several new possibilities:

  • Eliminating the BIOS and running steckOS directly from ROM.
  • Upgrading steckOS in-circuit and from within itself using a flash write tool.
  • Adding a ROM-based machine language monitor.
  • ROM BASIC/FORTH
  • Implementing demos and diagnostic tools.