71m6521be Maxim Integrated Products, Inc., 71m6521be Datasheet - Page 64

• Manufacturer Part Number: 71m6521be
• Description: Energy Meter Ic
• Manufacturer: Maxim Integrated Products, Inc.

The RX pin should be pulled down by a 10kΩ resistor and additionally protected by a 100pF ceramic capacitor, as shown in
Figure 33.
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.
Page: 64 of 97
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.
© 2005-2008 TERIDIAN Semiconductor Corporation
71M6521BE
71M6521BE
Figure 33: Connections for the RX Pin
TX
TX
RX
RX
Optical Interface
UART0 (TX/RX)
100pF
100pF
10kΩ
10kΩ
TX
TX
RX
RX
Energy Meter IC
71M6521BE
DATA SHEET
JANUARY 2008
V1.0