DS_6612_001
78M6612 Data Sheet
3.10 UART1 Interface
The pins TX1 and RX1 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 35 shows the basic connections. The TX1 pin
becomes active when the I/O RAM register TX1DIS is set to 0.
The polarity of the TX1 and RX1 pins can be inverted with configuration bits TX1INV and RX1INV,
respectively.
The TX1 output may be modulated at 38 kHz when system power is present. Modulation is not available
in BROWNOUT mode. The TX1MOD 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
TX1 is low for 6.25% of the period.
The receive pin (RX1) 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.
V3P3SY
R1
78M6612
10kΩ
100pF
RX
Phototransisto
V3P3SY
LED
R2
TX1
Figure 35: Connection for Optical Components
3.11 Connecting V1 and Reset Pins
A voltage divider should be used to establish that V1 is in a safe range when the power and
measurement unit is in mission mode (V1 must be lower than 2.9 V in all cases in order to keep the
hardware watchdog timer enabled). For proper debugging or loading code into the 78M6612 mounted
on a PCB, it is necessary to have a provision like the header shown above R1 in Figure 36. 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 8 to 10 kΩ in order to provide hysteresis
for the power fault monitor.
Rev. 1.2
71
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
