71M6543F/H and 71M6543G/GH Data Sheet
for UART1. The OPT_TX pin becomes active when the control field OPT_TXE (I/O RAM 0x2456[3:2]) is
set to 00.
The polarity of the OPT_TX and OPT_RX pins can be inverted with the configuration bits, OPT_TXINV
(I/O RAM 0x2456[0]) and OPT_RXINV (I/O RAM 0x2457[1]), respectively.
The OPT_TX output may be modulated at 38 kHz when system power is present. Modulation is not
available in BRN mode. The OPT_TXMOD bit (I/O RAM 0x2456[1]) enables modulation. The duty cycle is
controlled by OPT_FDC[1:0] (I/O RAM 0x2457[5:4]), 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 BRN mode is desired, the external components should be connected to V3P3D. However,
it is recommended to limit the current to a few mA.
V3P3SYS
R1
71M6543
OPT_RX
10 kΩ
100 pF
Phototransistor
V3P3SYS
LED
R2
OPT_TX
Figure 35: Connection for Optical Components
4.10 Connecting the Reset Pin
Even though a functional meter does not necessarily need a reset switch, it is useful to have a reset
pushbutton for prototyping as shown in Figure 36, left side. The RESET signal may be sourced from
V3P3SYS (functional in MSN mode only), V3P3D (MSN and BRN modes), or VBAT (all modes, if a battery is
present), or from a combination of these sources, depending on the application.
For a production meter, the RESET pin should be protected by the by the external components
shown in Figure 36, right side. R1 should be in the range of 100Ω and mounted as closely as possible
to the IC.
Since the 71M6543 generates its own power-on reset, a reset button or circuitry, as shown in Figure 36, is
only required for test units and prototypes.
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