Digital to OI Analog

[Mike Copioli]

Originally Posted by marcan

Not a problem - all we need to do is calibrate to our specific parts.


You are overestimating the amount of power a small MCU requires. Developing a USB Host device is much more complex than SPI which is a very simple protocol - a small PIC will do, which will definitely use less than 100mA of power. In Sleep mode (which is what the PIC should be most often in, if it doesn't have much to do and it's coded properly), the current is measured in single-digit microamps (yes, micro). With EVERYTHING enabled (not likely), running at 48Mhz (maximum, overkill) frequency, the maximum current drawn is 50mA. With a much more common 4Mhz frequency (plenty for most usages), current is 6mA. These values are for a PIC18F4550, which is a pretty big PIC. The digital pots use less than a milliamp each even under active conditions. The OI provides 100mA. You do the math.

Also, if we really wanted to use a high-current custom circuit, we could just buy the USB-Chicklet, and use it and a USB PIC to get 500mA.

Actually you are over estimating the power output of the joystick port. I'm not quite sure were the 100mA is coming from. I assure you it is not from the joyports.


LED Drivers (see pinouts on pages 7-10)
PORT 1 and PORT 3 each provide four LED output drivers. These LED drivers allow the connection of external LEDs that duplicate the function of the top eight Robot Feedback LEDs on the Operator Interface. The LED drivers provide 5V that is current limited to 10mA. Connect the LED’s anode to
the desired LED drive pin. Connect the LED’s cathode to any ground pins.

The LED drivers are the only way for you to get Power. They are limited to 10mA. If you are thinking of using the 5Vaux pin, that is limited to a value in uA. So unless you are sleeping your processor 90% of the time, you can not get enough power to run your device. Trust me, If there was a way to get power off of the O/I we would have. I guess if you can get your power consumption below 10mA you would be ok. But you still have to deal with the abysmal tolerances of the MCP42100.