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Callaloo Radio System: Part 2 – Building a Homebrew USB Device

by silentbicycle on 5/17/14, 5:03 PM with 4 comments

• by zokier on 5/17/14, 7:14 PM

  I'm not sure if attiny+ftdi really makes sense anymore these days. Checking digikey for prices (single units), that attiny84 costs $1.7 and the ftdi chip is $2, combined $3.7. Meanwhile you can get USB equipped PICs for $1.5 and ARM chips from Freescale and NXP starting from $3.

  Of course there is the complexity argument that goes both ways; with attiny+ftdi you need to fiddle with soft-uart and need to design with two chips (with supporting circuitry) instead of one. With ARM you are dealing with more complex MCU and USB is not abstracted away in some blackbox ftdi chip.

• by mikepurvis on 5/17/14, 7:24 PM

  Funny thing about the UART not being able to do manchester coding— since he's defining the protocol, it totally could! He'd just have to define each data byte as two wire bytes, and then translate on each end.

  On the whole though, it seems weird to eschew software USB but then go ahead with software serial. I would definitely have just done this as FTDI -> SPI. Or thrown away the AVR, and gone with a some dirty-cheap 32-bit ARM chip that has the needed peripherals built right in. Maybe an STM32F04, which is $3.64 at quantity 1:

  http://www.digikey.ca/product-detail/en/STM32F042K6T6/497-14...

  That's a 32-pin LQFP package, which is straightforward to hand-solder for an intermediate.