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TC1796
System Units (Vol. 1 of 2)
Clock System and Control
User’s Manual
3-4
V2.0, 2007-07
Clock, V2.0
3.2.1
Main Oscillator Circuit
The oscillator circuit, a Pierce oscillator, is designed to work with both, an external crystal
oscillator or an external stable clock source. It basically consists of an inverting amplifier
and a feedback element with XTAL1 as input, and XTAL2 as output.
When using a crystal, a proper external oscillator circuitry must be connected to both
pins, XTAL1 and XTAL2. The crystal frequency can be within the range of 4 MHz
to 25 MHz. Additionally are necessary, two load capacitances
C
X1
and
C
X2
, and
depending on the crystal type, a series resistor
R
X2
to limit the current. A test resistor
R
Q
may be temporarily inserted to measure the oscillation allowance (negative resistance)
of the oscillator circuitry.
R
Q
values are typically specified by the crystal vendor. The
C
X1
and
C
X2
can be used as starting points for the negative
resistance evaluation and for non-productive systems. The exact values and related
operating range are dependent on the crystal frequency and have to be determined and
optimized together with the crystal vendor using the negative resistance method.
Oscillation measurement with the final target system is strongly recommended to verify
the input amplitude at XTAL1 and to determine the actual oscillation allowance (margin
negative resistance) for the oscillator-crystal system.
When using an external clock signal, the signal must be connected to XTAL1. XTAL2 is
left open (unconnected). The external clock frequency can be in the range of 0 - 40 MHz
if the PLL is bypassed, and 4 - 40 MHz if the PLL is used.
The oscillator can also be used in combination with a ceramic resonator. The final
circuitry must be also verified by the resonator vendor.
shows the recommended external oscillator circuits for both operating
modes, external crystal mode and external input clock mode.