71M6531D Datasheet, PDF (66/115 Pages) Teridian Semiconductor Corporation – Energy Meter IC

Data Sheet 71M6531D/F-71M6532D/F
71M6531D/F, 71M6532D/F
RX
100 pF 10 kΩ
RX
FDS 6531/6532 005
TX
TX
Figure 34: Connections for UART0
3.9 Optical Interface (UART1)
The OPT_TX and OPT_RX pins can be used for a regular serial interface (by connecting a RS-232 tran-
sceiver for example), or they can be used to directly operate optical components (for example, an infrared
diode and phototransistor implementing a FLAG interface). Figure 35 shows the basic connections for
UART1. The OPT_TX pin becomes active when the I/O RAM register OPT_TXE is set to 00.
The polarity of the OPT_TX and OPT_RX pins can be inverted with the configuration bits, OPT_TXINV
and OPT_RXINV, respectively.
The OPT_TX output may be modulated at 38 kHz 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 OPT_RX pin uses digital signal thresholds. It 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.
V3P3SYS
71M6531D/F or 71M6532D/F
OPT_RX
100 pF 10 kΩ
R₁
Phototransistor
V3P3SYS
OPT_TX
R₂
LED
Figure 35: Connection for Optical Components
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