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A major goal of OT-Hobbies was to get around the limitations inherent in Arduino boards with the common -UNO form factor. These include:
1. too hard to make "permanent" connections to the Arduino female header pins.
You will notice that most examples of connection to the Arduino headers involve using jumpers with circular male pins, and connecting to whiteboards. Unfortunately, this is a 100% temporary solution to making connections.
A much better, and permanent, way to deal with this situation is to use a prototyping board with more convenient headers, especially the 3-row male header designs found on other embedded controller boards. Such 3-row headers typically use a signal-power-ground arrangement, and R/C servos as well as many sensors have female headers that plug directly into these. Also, the middle row on the headers is a convenient way to provide power to external devices, so you have both signal and power in a compact cable design, rather than something that looks more like a bowl of spaghetti wires.
2. poor customizability.
Essentially all Arduino boards are hardwired, and the only way to get custom functions is to buy a special-purpose shield, like an ethernet shield, or else buy a prototyping shield and wire up your own parts. There's not much way around this for really specialized circuitry, but it's a big PITN for the basics, such as (a) I/O pin protection, or (b) simple signal conditioning.
Good Customizability is a major feature of OT-Hobbies boards. The OT-Proto1 Shields are customizable in many different ways. The OT-Tredici boards allow custom resistor wiring on all I/O pins for interfacing different external voltages and loads, and also allows the ATmega chip to run at either 5V or 3.3V, as best suits applications. There are also jumpers for XBee signal routing, plus space for an auxiliary crystal, SPI SRAM or EEPROM chip, and beeper, plus the Extension Headers and I2C buss.
To deal with this, there are open-pads for adding series-R protection on I/O pins, as well as voltage-dividers on A/D channel pins and selected digital pins. The voltage-dividers make it easy to interface voltages outside the usual operating range (ie, 0..5V). Furthermore, OT-Hobbies Proto Shields have prototyping areas where all manner of additional circuitry can be wired on.
3. lack of protection against overvoltages and short-circuits on I/O lines.
The single most effective way to protect I/O pins on Arduino chips is with small-valued series-resistors, eg 150-330 ohms. These both limit maximum current out of the pins, and also in conjunction with the internal clamping diodes, greatly protect Arduino I/O pins from voltages outside the 0..Vcc range.
4. too little 5V and 3.3V current available - (especially at 3.3V).
This is a serious problem with many Arduino board designs. First off, some boards get their 3.3V regulated power from the USB chip, and this provides a barely usable 50 mA or so. Secondly, many boards use tiny SOT-223 and SOT-23 surface-mount voltage regulators, and these can overheat at relatively low current loads.
Eg, for a 5V regulator, if 12V is applied at Vin and the load current is just 200 mA, power dissipation is Pd = (12V - 5V) * 0.2A = 1.4 Watts, and this already exceeds the specs for the SOT-223 part by about 3X, resulting in serious overheating. See the regulator power dissipation page for details.
A more useful solution is to use larger TO-220 and DPAK regulators as found on OT-Hobbies boards. We also have space for a heatsink on the TO-220 5V regulator, for added heat dissipation.
6. too little internal RAM.
ATmega328 chips as found on -UNO style boards have only 2 KBytes of internal SRAM, which is easily exceeded in non-trivial sketches.
To deal with this problem, OT-Hobbies boards were designed to either use a larger processor, like ATmega1284 with 16 KBytes of SRAM, and/or with a DIP8 socket so an external SPI SRAM chip, like the 23LC1024 with 128 KBytes of RAM, can be added. This socket will also accept an SPI EEPROM chip, such as the 25LC1024 with 128 KByte of storage.
7. too few A/D channels.
ATmega328 chips have 6 A/D converter channels, but 2 of these are co-opted if the I2C buss is also used, leaving only 4 A/D channels left.
The OT-Hobbies Proto1 Shield was designed so extra ADC chips can be added to the prototyping area. Eg, this could be an MCP3208 chip which provides 8/ea 12-bit resolution A/D channels and a sampling rate of 100 Ksps, which is 4X the resolution and 2.5X faster than the Arduino chips. This chip can be controlled by the Arduino-UNO SPI port, and its analog input channels connected to the "Extension Header" pins. This leaves the original 6 channels of the Arduino-UNO still available.
In addition, the OT-Tredici board already has 8 A/D channels as well as a totally separate 3-place I2C buss.
8. presence of only one RS232 hardware UART.
The ATmega328 processor on -UNO style boards has only 1 "hardware" UART, which is a serious limitation. It's not easy to both interface to external RS232 devices and perform debugging (via the USB port) simultaneously. The Arduino IDE does provide a "software" UART library, but this is barely usable for serious work.
In contrast, the ATmega1284 processor on the OT-Tredici board has 2 "hardware" UARTs, so for instance, you can perform debugging via USB at the same time as conducting high-baudrate communications with an XBee transceiver, and with no problems as seen with the software UARTs.
9. problems with poor level-shifter design of many XBee shields.
There are many XBee shield designs on the market, but almost all of them have some weak design points, either with not providing enough 3.3V power to operate the transceivers, or problems with the 5V-to-3.3V level-shifter circuitry design. Many of these boards can damage the 3.3V inputs pins on the XBee modules. See the XBee design issues page for more info.
All of the OT-Hobbies boards have high-current 3.3V regulators, as well as 5V-to-3.3V level-shifter designs that will not damage the 3.3V XBee input pins.