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The next IDIOTic item is the parasitically powered Inverted MIDI Slave Unit.

This unit can be used for lots of different purposes ranging from just a simple status monitor or a device to control lights and the light's brightness, pan and tilt or to drive more complex items such as musical synthesizers and large public displays.

The picture above shows it displaying received MIDI messages. Great for testing and diagnostics.


MIDI (Music Instrument Digital Interface) is a current loop interface used for connecting electronic musical instruments and control devices together.

MIDI uses optocouplers to provide electrical isolation between devices.

It is just a 31,250 bps serial communication over a current loop.

The odd 31,250 bit rate was selected to keep costs low (back in the 1980's) by enabling its timebase to be derived from a 1MHz clock source that is divided by 32.

1,000,000/32 = 31,2500

The specification stipulates that it uses a 5mA current loop.

It uses current on to represent a logical '0' and an current off for a logical '1'.

The attached slave unit uses an optocoupler such as a Sharp PC900 to convert theses current states into logic level voltages that can then be decoded using standard UART.

The current control is done via a 5V voltage serially feeding two 220 ohm resistors in the master unit and a 220 ohm resistor and optocoupler diode at the Slave.


With this arrangement when idle there is no current passing and hence no energy to harvest. This the opposite to standard industrial and TTY current loops.

By inverting the signalling arrangement (to align with standard current interfaces) we can provide harvestable power idle state.

If desired electrical isolation can be retained with the addition of an additional power supply to provide isolated power in the slave device.



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This is a picture of the MIDI interface PCB made for the STK3700.

It can been seen populated and connected in the first picture of this article.


The Current Loop energy harvester is connected in series in the current loop replacing the slave's 220 ohm resistor.

With the diode drop energy harvesting system the available power is about

P = VI

0.6 x 0.005 = 3mW.


This is enough to power to initialise and operate EFM32 on a periodic basis.

To operate on a continuous or more frequent basis more power is required.

This can be addressed from several different fronts;

     1. The inverted MIDI signalling can have its current increased by the adjustment of the inline current control resistors.

     This current should not be increased to more than 20mA to exceed the Energy Harvester's rating. I am sucessfully using 15mA (for 9mW).


     2. Power consumption of the slave unit be reduced.

     The largest burden is the optocoupler. It is currently drawing 2.3mA (@VMCU).

     It can be replaced with a digital isolator that draws about 0.3mA or other feasible option.


     3. The use of the LEUART is not an option.

     The LEUART is limited to 9600 baud and MIDI operates at 31,2500 baud thus require the unit to operate with a standard UART in Energy Mode 1.