71M6521DE Datasheet, PDF (71/107 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/DH/FE Data Sheet
Even though a functional meter will not necessarily need a reset switch, it is useful to have a reset pushbutton for
prototyping, as shown in Figure 40, left side. The RESET signal may be sourced from V3P3SYS (functional in
MISSION mode only), V3P3D (MISSION and BROWNOUT modes), VBAT (all modes, if 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 external components shown in Figure 40, right side. R₁ should be in the range of 100Ω and mounted
as closely as possible to the IC.
Since the 71M6521DE/DH/FE generates its own power-on reset, a reset button or circuitry, as shown in
Figure 40, left side, is only required for test units and prototypes.
VBAT/
V3P3D
R₂
1kΩ
Reset
Switch
1nF
V3P3D
71M6521
10kΩ
R₁
RESET
DGND
71M6521
100Ω
R₁
RESET
DGND
Figure 40: External Components for RESET: Development Circuit (Left), Production Circuit (Right)
Connecting the Emulator Port Pins
Capacitors to ground must be used for protection from EMI. Production boards should have the ICE_E pin connected
to ground.
If the ICE pins are used to drive LCD segments, the pull-up resistors should be omitted, as shown in Figure 41, and
22pF capacitors to GNDD should be used for protection from EMI.
It is important to bring out the ICE_E pin to the programming interface in order to create a way for
reprogramming parts that have the flash SECURE bit (SFR 0xB2[6]) set. Providing access to ICE_E ensures that
the part can be reset between erase and program cycles, which will enable programming devices to reprogram the
part. The reset required is implemented with a watchdog timer reset (i.e. the hardware WDT must be enabled).
V3P3D
LCD Segments
(optional)
71M6521
62Ω
62Ω
ICE_E
E_RST
E_RXTX
62Ω
22pF 22pF 22pF
E_TCLK
Figure 41: External Components for the Emulator Interface
Crystal Oscillator
The oscillator of the 71M6521DE/DH/FE drives a standard 32.768kHz watch crystal. The oscillator has been
designed specifically to handle these crystals and is compatible with their high impedance and limited power handling
capability. The oscillator power dissipation is very low to maximize the lifetime of any battery backup device attached
to VBAT.
Rev 2
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