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Electronics -- USB 8bit Interface Board

About this Design

The goal was to make a small USB interface board which could easily be connected to microcontroller boards. It should support USB-powered as well as externally-powered operation. External components are kept at a minimum (but there are still quite a lot of them).

Electronic Circuit

Download USB 8bit interface board circuit schematic:
PNG image (734x999 as seen below): usb8bit.png (25kb)
High-quality PDF: usb8bit.pdf (81kb)
Permission to copy and use this schematic is hereby granted provided credit is given where it is due.

USB 8bit interface board circuit schematic [25kb]

The key parts of the design were combined from the FTDI's data sheed about the FT245BM USB FIFO chip.
The EEPROM is actually too big (you may use a 93C46 type as well but I had the '56 around).
The board may be used as USB-powered device (externally connect pins 1 and 2 on the SUPPLY pin header) and as externally-powered (leave pin 1 open and apply 5V to pins 2 and 3).


prototype image (top) [5kb]
[click to enlarge: 65kb JPEG]

As the FT245BM is an SMD chip (32LD LQFP; 0.8mm pitch), it was necessary to create a PCB for the design. You can see the prototype on the left: I designed it to be small but without requiring any more SMD components besides the main IC.

As I did not have the 27 Ohm resistors (R3 and R4) around that weekend, I used two 47 Ohm in parallel (yielding 23.5 Ohm). [That's why the resistors behind the EEPROM IC look so strange.] I added C7 and C8 to the design after building the board, hence the 10u decoupling cap C8 was soldered directly on the bottom (and C7 is not yet on the board).

As can be seen on the second image, this SMD can be real pain to solder especially when the design is dense and a ground plane is used...

prototype image (bottom) [6kb]
[click to enlarge: 81kb JPEG]

Rev. 4 SMD Board

Rev. 4 SMD image (top) [8kb]
[click to enlarge: 117kb JPEG]

With all the necessary parts at hand and a commercially manufactured PCB, things clearly look much nicer. I re-designed the above board using SMD parts for most things which made it again smaller. The used board area is now 2.5 x 4cm.

Application: The USB8Bit board is flipped bottom-up and then sticked directly on top of another board (normally one with a microcontroller on it). Normally only 4 of the 7 handshake lines are needed.

The LED and it's associated resistor are not mounted on this board.

Rev. 4 SMD image (bottom) [9kb]
[click to enlarge: 150kb JPEG]

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Last modified: 2006-08-19 14:43:27