Monday, January 4, 2016

Supported Hardware

Currently the MMDVM only runs on the Arduino Due. This is an ARM Cortex-M3 processor from Atmel which is clocked at 80 MHz. It has two ADCs and two DACs although only one is used for the MMDVM. It is a well supported platform and the only one I was aware of when I started, this was due to an interesting article in the RSGB’s RadCom about using a Due with a TFT display as a means of visualising the output of a receiver using an FFT. While the code and hardware behind this project was not of interest, the fact that it had enough processing power to perform this task was. A blog entry from M0XPD showed a Due acting as a high performance audio filter, yet again none of the code or hardware was copied, but the idea proved that the built-in hardware could do a suitable job of processing audio with a little amount of analogue electronics.
The electronics consists of a few op-amps for low pass filtering and a few switches for controlling the transmitter and LEDs. Or basic testing a couple of resistors and capacitors can be used, but putting such a system actually on-air would be unwise due to unwanted output frequencies particularly from the DAC (the sample rate of 24 kHz could be troublesome).
Since the Due development started, I was made aware of the Teensy 3.1/3.2 and later some of the STM32 series of devices. These mostly use ARM Cortex-M4 devices with various peripherals. The Teensy clocks at 72 MHz and the fastest STM32 is 188 MHz! The Teensy is cheaper than the Due, but its USP is its size, it is very small indeed. Wouldn’t it be cool to have a repeater that can fit into the palm of your hand easily barring the RF bits? The STM32s are very very cheap but seem to lack a decent USB serial hardware interface available to any running program. The Teensy can be developed using the same Arduino GUI as the Due while the STM32 can either be developed using MBED with a plethora of GUIs, or with STM32Duino. I am not looking at these platforms explicitly as I am busy with the actual data protocols, but others are. I await their results with interest.

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