The USB Project Board includes a USB-programmable 18-pin PIC18F24K50 microcontroller and a ULN2803 power output chip, giving students and hobbyists a rugged miniature input/output board that can be embedded in a wide variety of projects such as motors, lamps, and solenoids. The ULN2803 power output device provides eight outputs each of which can deliver around 500mA with an appropriate power supply. Eight general-purpose input/output microcontroller pins are also provided, and these are protected with resistors.
Programming is carried out through the USB port The bootloader is open-source, so other USB-programmable projects can be based on it. Files can be sent to the chip using the free mLoader utility. The board also is compatible with Flowcode 6.
The board is powered normally at 5V from a standard USB socket or a USB hub. Alternatively, it can be powered from an external source of 2.3V to 5.5V; a jumper is on board to select USB or External power source. The bootloader for the on-board PICmicro chip is based on open-source Microchip code. The board includes a resettable fuse and other components which offer protection for the board and your PC's USB circuitry. The holes on the board are compatible with standard screw terminal connectors which can be soldered in if required. A USB cable is not included.
The board terminals are marked with their microcontroller pin designations, namely A0-A4, C0-C2, C6 and C7. The microcontroller I/O ports have a 270Ω resistor in series to provide a level of protection. This has the added advantage that the outputs can be used to drive things like LEDs, in which case the output current is limited by the resistor to around 10mA. Additional series resistors can be introduced to limit the current further if this is required.
The provision of the on-board ULN2803A driver means that higher voltage and current outputs can be achieved in addition to the 5V microcontroller I/O interfaces. The microcontroller's port B0 to B7 outputs drive the eight ULN2803A channel outputs, and are provided on terminals marked M0 to M7. These outputs can sink up to 500mA from an externally provided power source. The device also provides integral suppression diodes for inductive loads such as motors and solenoids. When inductive loads are being driven, simply connect the external common positive power rail to the terminal marked CL.
Allowable Voltage/Current Levels
- Maximum individual sink current for transistor drivers on terminals M0 to M7: 500mA
- Maximum total sink current for all transistor drivers on terminals M0 to M7: 1200mA
- Maximum off-state voltage to transistor drivers on terminals M0 to M7: 50V
- Maximum output current sink or source by any microprocessor I/O pin: 25mA
- Maximum total current sunk by all microprocessor ports: 200mA
- Maximum total current sourced by all microprocessor ports: 185mA
Programming with Bootloader The microprocessor is pre-programmed with a bootloader, which sets the internal clock active at 48MHz and flashes the Green LED awaiting program download from mLoader and the host PC USB connection. This simplifies programming from a Flowcode-created program. After approximately four seconds, the LED will stop flashing and any previously downloaded user program will run. The LED flash period is reduced to approximately one second if no USB host is detected. Pressing the on-board Reset button causes the bootloader to restart.
USB Project Board Resources
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