Power, Reset, and Mode Selection Lines

Pin Functions and Descriptions

* Implemented in (supported through) firmware

Line functions defined by the Application Firmware are shown in blue.

Power Supply Considerations

The EM203 must be powered by a stabilized DC power supply with a nominal output voltage of 5V ± 5% tolerance. The typical current consumption of the EM203 is approximately 220mA in 100BaseT mode.

Power Supply Circuit

Many power supply designs are compatible with EM203. Below is a circuit used by Tibbo, capable of handling input voltages in the range of 9–24V.

• Voltage Regulator (U1): The regulator in this circuit is a widely used power IC manufactured by ON Semiconductor.

• Capacitors (C1 and C2): Avoid surface-mount capacitors; instead, use through-hole aluminum capacitors to minimize noise from the power supply.

• PCB Layout: This is an analog circuit, so proper layout is critical. Ensure good practices for noise reduction and signal integrity.

• Testing: Use an oscilloscope to observe the power supply’s waveform under various conditions, including low and high input voltages, as well as light and heavy loads. Fine-tune the circuit as necessary for optimal performance.

External Reset Considerations

Proper external reset circuitry is essential. The reset pulse must be active HIGH, and we strongly discourage using low-cost RC networks or other unreliable methods for generating the reset signal. The reset should remain active 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. The reset pulse should last at least 50 ms from the moment the power supply voltage exceeds 4.6V.

If the EM203 is used as a communications co-processor within a larger system that includes its own CPU, the host microcontroller can control the EM203's RST line through a general-purpose I/O pin. Since most microcontroller I/O pins default to HIGH at power-up, the reset will automatically be applied to the EM203 during the microcontroller’s reset cycle. The host microcontroller can release the EM203 from reset by switching the I/O line state to LOW at the appropriate time.

MD Line and Operating Modes

The MD line of the EM203 determines its operating mode and interacts with its application firmware. Its functionality is as follows:

1. At Power-Up:
• If MD is HIGH, the EM203 verifies and executes the application firmware stored in its internal flash memory.
• If MD is LOW, the EM203 enters serial upgrade mode.

1. During Operation:
• When the serial-to-Ethernet firmware is running, the MD line can be used to switch the EM203 into serial programming mode.

1. Integration with a Host System:
• In host-controlled systems, the MD line can be managed by the host microcontroller, along with the RST line. This capability allows the host microcontroller to switch between the EM203's operating modes as needed.