Power, Reset, and Mode Selection Lines
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The EM500 should be powered from a stabilized DC power supply with a nominal output voltage of 3.3V (+/- 5% tolerance). The module's current consumption is approximately 260mA. Providing an adequate power supply is very important -- a poorly built circuit may affect the EM500's operation. We recommend that you use a switching power supply. One example of such a circuit is shown below.
Proper external reset is a must! Reset pulse should be active LOW. We strongly advise against using low-cost RC circuits and other unreliable methods of generating reset pulses. Reset should be applied for as long as the power supply voltage is below 2.9V. We recommend using a dedicated reset IC, such as the MCP130-300 device from Microchip. This part has a trip point at ~2.95V -- perfect for the EM500.
If the EM500 is to serve as a communications co-processor in a larger system that has its own CPU (microcontroller) it is also OK to control the RST line through a general-purpose I/O pin of this CPU. Reset pulses for the EM500 can then be generated programmatically, by setting the I/O pin of the CPU to LOW and then to HIGH.
The function of the MD line is described in Appendix 2: Setup Button (MD line).
Power supply circuit
Many power supply circuits will work well. The one below is being used by Tibbo. This circuit can handle input voltages in the 9-24V range.
•U1 (AP1501-33) is a popular power IC manufactured by Anachip (now Diodes Incorporated, www.diodes.com)
•C1 and C2 capacitors: Do not use SMD capacitors -- use regular through-hole aluminum capacitors. This really helps reduce the noise produced by the power supply.
•This is an analog circuit, so layout matters. Apply reasonable "good layout" effort.