FDS 6531/6532 005
Data Sheet 71M6531D/F-71M6532D/F
3.10 Connecting the V1 Pin
A voltage divider should be used to establish that V1 is in a safe range when the meter is in MISSION
mode (see Figure 36). V1 must be lower than 2.9 V in all cases in order to keep the hardware watchdog
timer enabled. The resistor divider ratio must be chosen so that V1 crosses the VBIAS threshold when
V3P3 is near the minimum supply voltage (3.0 VDC). A series resistor (R3) provides additional hystere-
sis, and a capacitor to ground (C1) is added for enhanced EMC immunity.
The amount of hysteresis depends on the choice of R1 and R3: If V1 < VBIAS, approximately 1 µA will
flow into the on-chip V1 comparator causing a voltage drop. If V1 ≥ VBIAS, almost no current will flow
into the comparator. The voltage drop will require V3P3 to be slightly higher for V1 to cross the VBIAS
threshold when V3P3 is rising as compared to when V3P3 is falling. Maintaining sufficient hysteresis
helps to eliminate rapid mode changes which may occur in cases where the power supply is unstable with
V1 close to the VBIAS threshold point.
R1
R3
V3P3
GND
16.9kΩ
V1
20kΩ
C1
100pF
R2
Figure 36: Voltage Divider for V1
3.11 Connecting the Reset Pin
Even though a functional meter will not necessarily need a reset switch, it is useful to have a reset push-
button for prototyping as shown in Figure 37, left side. The RESET signal may be sourced from
V3P3SYS (functional in MISSION mode only), V3P3D (MISSION and BROWNOUT 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 37, right side. R1 should be in the range of 100 Ω and mounted as closely as possible to the IC.
VBAT/
V3P3D
V3P3D
71M6533
71M6532D/F
71M6531D/F
71M6531D/F
71M6532D/F
R2
1kΩ
Reset
Switch
RESET
RESET
DGND
100Ω
R1
10kΩ
R1
0.1µF
DGND
Figure 37: External Components for the RESET Pin: Push-button (Left), Production Circuit (Right)
Since the 71M6531 generates its own power-on reset, a reset button or circuitry, as shown in Figure 37,
is only required for test units and prototypes.
3.12 Connecting the Emulator Port Pins
Even when the emulator is not used, small shunt capacitors to ground (22 pF) should be used for protec-
tion from EMI as illustrated in Figure 38. Production boards should have the ICE_E pin connected to
ground.
v1.2
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