Just a quick status update on the isostick. Presently the isostick uses the Atmel AVR32 UC3A3 microcontroller, which is an excellent chip indeed! However, the maximum CPU and bus frequency for the AVR32 UC3 is 66MHz, and the maximum MCI (SD/MMC Controller) clock speed is half that, so 33MHz. Each clock cycle the microSD cards transfer 4bits, so 8bits (1 byte) every 2 cycles, or 33/2 = 16.5MByte/s theoretical transfer speed. I see real-world transfer speeds around 12.5MByte/s.
The Atmel SAM3U series of ARM Cortex-M3 micros caught my eye early in the development process when first hunting for a suitable MCU. They remained in the sampling phase for quite some time however, and I decided to go with the aforementioned AVR32. Recently the SAM3U has gone into production and begun popping up at the usual distributors.
Why go SAM3U? It’s faster! I expect to see real-world transfer speeds of around 20MByte/s on the SAM3U-based design, which is almost double what I’m seeing on AVR32. Note that the SAM3U can run the microSD card at 48MHz, and the SD Specification 2.0 has a maximum of 50MHz, so it doesn’t get much faster than that! The SAM3U is also less expensive in volume and physically smaller, making the PCB layout much less cramped.
What problems may arise? The code may be difficult to port in the time I have allotted. It is highly abstracted, however, and I feel it can be fully ported in a week’s time. Failing that, I will continue forward with the AVR32 solution, for which beta (and barring any hardware flaws, production) PCBs are ready to go. An eval kit is on its way to me as I write this, and should arrive on Monday, April 4th. In the mean time I will continue work on the AVR32 firmware and investigate points of difficulty for the port.
What does this mean for you? I won’t delay the beta more than 2 weeks to investigate this. I plan on having the beta boards in hand and ready to distribute by the end of April, worst-case. The final board design using the AVR32 is done and ready to be used in case I do not use the SAM3U.
Thank you all for your patience and understanding as I refine the isostick design!