78M6610+LMU Data Sheet
Clock Management
The device can be clocked by either a trimmed internal RC oscillator or by oscillator circuitry that relies on
an external crystal. The internal RC oscillator provides an accurate clock source for UART baud rate
generation. Only time based calculations such as line frequency and watt-hour (energy) are affected by
clock accuracy.
The chip hardware automatically handles the clock sources logic and distributes the clock to the rest of
the device. Upon reset or power-on, the device will utilize the internal RC oscillator circuit for the first 1024
clock cycles, allowing the external crystal adequate time to start-up. The device will then automatically
select the external clock, if available. It will also automatically switch back to the internal oscillator in the
event of a failure with the external oscillator. This condition is also monitored by the processor and
available to the user in the STATUS register.
The 78M6610+LMU external clock circuitry requires a 20.000MHz crystal. The circuitry includes two 18pF
ceramic capacitors. The figure below shows the typical connection of the external crystal. This oscillator is
self biasing and therefore an external resistor should NOT be connected across the crystal.
18pF
XIN
20.000MHz
XOUT
18pF
78M6610+LMU
Figure 6. Crystal Connections
An external 20MHz system clock signal can also be utilized instead of the crystal. In this case, the
external clock should be connected to the XOUT pin while the XIN pin should be connected to GNDD.
Alternatively, if no external crystal or clock is utilized, the XOUT pin should be connected to GNDD and
the XIN pin left unconnected.
13
Rev 0
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