Power, Reset, and Mode Selection Lines
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* Implemented in (supported through) firmware
The EM203 should be powered from a stabilized DS power supply with output nominal voltage of 5V (+/- 5% tolerance). Current consumption of the EM203 is approximately 220mA (in 100BaseT mode).
Proper external reset is a must! Reset pulse should be active HIGH. We strongly advise against using low-cost RC-networks and other unreliable methods of generating reset. Reset should be applied for as long as the power supply voltage is below 4.6V. We recommend using a dedicated reset IC with brownout detection, such as the MAX810. Reset pulse length should be no less than 50ms, counting from the moment the power supply voltage exceeds 4.6V.
If the EM203 is used to serve as a communications co-processor in a larger system that has its own CPU it is also OK to control the RST line of the EM203 through a general-purpose I/O pin of the "host" microcontroller. I/O pins of many microcontrollers default to HIGH after the powerup and this means that the reset will be applied to the EM203 in parallel with the reset of the microcontroller. All the host microcontroller has to do is release the EM203 from reset at an appropriate time by switching the state of the I/O line to LOW.
The MD line of the EM203 is used to select the operating mode of the EM203 and/or its application firmware. This pin has the following functionality:
When the EM203 is used as a co-processor in a host system the MD line can be also controlled by the host microcontroller. Ability to control both the RST and the MD lines will allow the host microcontroller to switch between the operating modes of the EM203.
Power supply circuit
Many power supply circuits will work well. The one below is being used by Tibbo. The circuit can handle input voltages in the 9-24V range.