AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 18

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

AN2321/D
Oscillators
18
approximately 5mA at 1V (V
source of the external capacitor, it is necessary to ensure a minimum pull-up
resistor value of 2k Ω .
A diode to clamp the voltage of the reset pin is also recommended to prevent
the voltage from exceeding the supply voltage and also to provide a faster
discharge for the capacitor when the supply is removed.
Many MCUs incorporate an inverting amplifier for use with an external crystal
or ceramic resonator in a Pierce oscillator configuration. The following
discusses the minimum gain (transconductance) of the amplifier required to
ensure oscillation with specific external components.
Figure 17
MCUs for crystals in the 1MHz to 20MHz frequency range. The circuit internal
to the MCU is shown in simplified form as a NAND gate followed by an inverter.
The NAND gate has two inputs; one is connected to the MCU pin called OSC1
and the other input is connected to the inverted internal STOP signal.
For the circuit to oscillate, there must be positive feedback and the closed-loop
gain must be greater than unity. Resistor R
increases the open-loop gain requirement of the amplifier. R
as large as possible to minimize the feedback whilst still overcoming leakage
currents at start-up. When using a crystal between 1MHz and 20MHz, R
should be in the range of 1M Ω to 10M Ω . For ceramic resonators, a R
is normally used.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
Figure 17. Standard Pierce Oscillator for 1MHz to 20MHz Operation
For More Information On This Product,
shows the typical standard Pierce oscillator configuration used on
External
components
Go to: www.freescale.com
STOP
OSC1
C
1
OL
R
Q
0
). Together with the low impedance voltage
OSC2
C
0
2
results in negative feedback which
Internal to MCU
0
is usually made
MOTOROLA
0
of 1M Ω
0