AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 18

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

S12 CRG — Pierce Oscillator
S12 CRG — Pierce Oscillator
500 kHz to 40 MHz crystal / resonator (not available on MC9S12D256x K36N and K79X masks).
On the D-family, pull XCLKS pin low at reset. This is also the configuration to select for using an external
2.5 V oscillator (square wave).
R
Values for R
Additional Pierce Oscillator layout considerations
A key requirement for the Pierce configuration is to minimize the parasitic loading between the EXTAL
and XTAL tracks. Keep the EXTAL track as short as possible and tracked away from the XTAL track.
18
B
is required to bias the oscillator into its correct operating region. 1 MΩ is a reasonable value for R
S
, C
1
and C
* R
2
Refer to manufacturer’s data.
should be confirmed with the crystal or resonator manufacturer.
S
can be zero (shorted) when used with higher frequency crystals.
Designing Hardware for the HCS12 D-Family, Rev. 0
Figure 5. Pierce Oscillator Configuration
MCU
EXTAL
XTAL
R
S
*
R
B
Crystal or
ceramic resonator
C
C
1
2
VSSPLL
Freescale Semiconductor
B
.