71M6521BE
Energy Meter IC
DATA SHEET
JANUARY 2008
UART0 (TX/RX)
The RX pin should be pulled down by a 10kΩ resistor and additionally protected by a 100pF ceramic capacitor, as shown in
Figure 33.
71M6521BE
10kΩ
100pF
RX
RX
TX
TX
Figure 33: Connections for the RX Pin
Optical Interface
The pins OPT_TX and OPT_RX can be used for a regular serial interface, e.g. by connecting a RS-232 transceiver, or they
can be used to directly operate optical components, e.g. an infrared diode and phototransistor implementing a FLAG interface.
Figure 34 shows the basic connections. The OPT_TX pin becomes active when the I/O RAM register OPT_TXDIS is set to 0.
The polarity of the OPT_TX and OPT_RX pins can be inverted with configuration bits OPT_TXINV and OPT_RXINV, re-
spectively.
The OPT_TX output may be modulated at 38kHz when system power is present. Modulation is not available in BROWNOUT
mode. The OPT_TXMOD bit enables modulation. The duty cycle is controlled by OPT_FDC[1:0], which can select 50%, 25%,
12.5%, and 6.25% duty cycle. A 6.25% duty cycle means OPT_TX is low for 6.25% of the period.
The receive pin (OPT_RX) may need an analog filter when receiving modulated optical signals.
With modulation, an optical emitter can be operated at higher current than nominal, enabling it to increase the distance
along the optical path.
If operation in BROWNOUT mode is desired, the external components should be connected to V3P3D.
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