Datasheet

DS_6612_001 78M6612 Data Sheet
Rev 2 25
3.9 UART1 (TX1/RX1)
The TX1 and RX1 pins can be used for a regular serial interface (by connecting a RS-232 transceiver for
example), or they can be used to directly operate optical components (for example, an infrared diode and
phototransistor implementing a FLAG interface).
3.10 Connecting V1 and Reset Pins
A voltage divider should be used to establish that V1 is in a safe range (see Figure 15). V1 must be
lower than 2.9 V in all cases in order to keep the hardware watchdog timer enabled. A series 5 kΩ
resistor (R3) and a capacitor to ground (C1) are added for enhanced EMC immunity. The parallel
impedance of R1 and R2 should be approximately 8 kΩ to 10 kΩ in order to provide hysteresis for the
power fault monitor.
Figure 15: Voltage Divider for V1
Even though a functional power and measurement unit will not necessarily need a reset switch, it is useful to
have a reset pushbutton switch for prototyping, as shown in Figure 16, left side. The RESET signal may be
sourced from V3P3SYS, or VBAT (if a battery is present), or from a combination of these sources, depending
on the application.
For production, the RESET pin should be protected by the external components shown in Figure 16, right
side. R
1
should be in the range of 100 Ω and mounted as closely as possible to the IC. The RESET pin
can also be directly connected to ground.
Figure 16: External Components for RESET: Development Circuit (Left), Production Circuit (Right)
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
R
1
RESE
T
78M6612
DGN
D
V3P3D
R
2
Reset
Switc
h
1k
Ω
1nF
10k
Ω
R
1
DGN
D
R
2
VBAT/
V3P3D
Reset
Switc
h
1k
Ω
1nF
10k
Ω
R
1
100
R
1
RESET
78M6612
DGND
100
Ω