Datasheet
71M6521DE/DH/FE Data Sheet
Page: 70 of 107 Rev 3
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 38 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,
respectively.
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.
Figure 38: Connection for Optical Components
Connecting V1 and Reset Pins
A voltage divider should be used to establish that V1 is in a safe range when the meter is in mission mode (V1 must
be lower than 2.9V in all cases in order to keep the hardware watchdog timer enabled). For proper debugging or
loading code into the 71M6521DE/DH/FE mounted on a PCB, it is necessary to have a provision like the header
shown above R1 in Figure 39. A shorting jumper on this header pulls V1 up to V3P3 disabling the hardware watchdog
timer.
The parallel impedance of R1 and R2 should be approximately 20 to 30kΩ in order to provide hysteresis for the
power fault monitor.
Figure 39: Voltage Divider for V1
OPT_TX
R
2
R
1
OPT_RX
71M6521
V3P3SYS
Phototransistor
LED
100kΩ
100pF
V3P3SYS
OPT_TX
R
2
R
1
OPT_RX
71M6521
V3P3SYS
Phototransistor
LED
100kΩ
100pF
V3P3SYS
V3P3
R
2
V1
R
1
R
3
5kΩ
C
1
100pF
GND
V3P3
R
2
V1
R
1
R
3
5kΩ
C
1
100pF
GND