Power, Reset, and Mode Selection Lines
The EM200 should be powered from a stabilized DC power supply with nominal output voltage of 5V (+/- 5% tolerance). Current consumption of the EM200 is approximately 220mA (in 100BaseT mode). Providing an adequate power supply is very important -- poorly built circuit may affect EM200 operation. We recommend that you use a switching power supply circuit. One (but not the only) example of such circuit is shown below.
Proper external reset is a must! Reset pulse should be an active HIGH. We strongly advise against using low-cost RC-networks and other unreliable methods of generating reset pulse. 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 MAX810. Reset pulse length should be no less than 50ms, counting from the moment the power supply voltage exceeds 4.6V.
If the EM200 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 EM200 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 proper reset will be applied to the EM200 when the host device is switched on. All the host microcontroller has to do is release the EM200 from reset at an appropriate time by switching the state of the I/O line to LOW.
The function of the MD line is described in Setup Button (MD line).
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 9-30V range.
U1 (MC35063) is a very popular power IC manufactured by ON Semiconductor.
R1 is very important. It is just 1 (one!) Ohm, but we really do not recommend the user to omit it.
R2 and R3 are "1% tolerance" (high-precision) because they define the output voltage of the power supply.
C1 and C3 capacitors: Do not use SMD capacitors -- use regular through-hole aluminum capacitors. This really helps reduce noise produced by the power supply.
This is an analog circuit, so layout matters. Apply reasonable "good layout" effort.
Ideally, one should use an oscilloscope to see what sort of "square wave" the power supply generates, both at low and high input voltages, as well as light and heavy loads. R1 can be adjusted to achieve a better (cleaner) square wave signal on a particular PCB layout. There are no recipes here -- just try and see what works for your circuit.