Thunderbird-yet another 68000 SBC

Introduction

In homebrew retro computing, the CompactFlash (CF) card is very popular, but SD cards are also used. In Thunderbird, both can be used.

CF/IDE-interface

• Easy to connect: The CF card uses the well-known IDE interface, meaning it can be connected directly to the 8- or 16-bit bus.
• Fast: It is fast due to the IDE interface. After issuing a few simple commands to read a sector, the BIOS only needs to read 512 bytes from the IO-port (Z80) or memory (68000).
• Obsolete: CF cards are no longer widely used, but they can still be easily ordered online.

SD-card

• Widely used: Despite its limitations, the SD card remains widely used.
• Easy to connect: Although the SD card operates at 3.3V, most adapters include a voltage converter to 5V. Because of the SPI interface, only four IO lines are needed (COPI, CIPO, CLK and CS). In the Thunderbird design, the spare input and output lines from the serial chip (TL16C550) are used for that task.
• Slow: In homebrew retro computers, there is usually no dedicated SPI interface. It can be simulated via bit-banging, but this is very, very slow (about 10–20 KB/s). However, this is comparable to the speed of an old-school floppy disk. In Fuzix, it gives you the feeling of an old timesharing Unix machine. Every command takes its time to execute, but after all it works good.

SD to 44 pin IDE interface

On the internet, you can find inexpensive ($10) SD-to-IDE converters. These tiny boards accept either a full-size or microSD card and feature a 44-pin IDE interface. This fits perfectly into the Thunderbird design! I tested it on the board, and it worked flawlessly. The interface supports both 16-bit and 8-bit modes, with the latter being ideal for Z80 or 6502 designs.

Conclusion

For me, the SD-to-IDE interface is a winner. Although there is a slight speed difference between the CF card and the SD-to-IDE interface, their speeds are approximately the same. However, since SD cards are more common, I prefer the SD-to-IDE interface.

Introduction

In this log, bugs and potential improvements for the Thunderbird system are described. Fortunately, only minor bugs have been found so far.

Bugs

• The CF/IDE busy indicator LED is operating in reverse. When a CF/IDE adapter is inserted, the LED lights up immediately, and when the device is busy, the LED turns off. 

Potential improvements

• The Thunderbird system, with the CF card running at full speed, draws almost 240mA. Therefore, it is advisable to use a larger polyfuse of 350mA or 500mA instead of the 250mA fuse.
• When reading a word from the IDE/CF card, the bytes must be swapped due to the little-endian/big-endian difference. This requires extra instructions, but a slight improvement can be achieved by handling it in hardware. Simply swap the low and high data lines at the CF/IDE interface.