LPC2131/2132/2138 User Manual - mct.net

Philips Semiconductors Preliminary User Manual
ARM-based Microcontroller
CRYSTAL OSCILLATOR
LPC2131/2132/2138
While an input signal of 50-50 duty cycle within a frequency range from 1 MHz to 50 MHz can be used by LPC2131/2132/2138
if supplied to its input XTAL1 pin, this microcontroller’s onboard oscillator circuit supports external crystals in the range of 1 MHz
to 30 MHz only. If on-chip PLL system or boot-loader is used, input clock frequency is limited to exclusive range of 10 MHz to 25
MHz.
The oscillator output frequency is called F osc and the ARM processor clock frequency is referred to as cclk for purposes of rate
equations, etc. elsewhere in this document. F osc and cclk are the same value unless the PLL is running and connected. Refer to
the PLL description in this chapter for details and frequency limitations.
Onboard oscillator in LPC2131/2132/2138 can operate in one of two modes: slave mode and oscillation mode.
In slave mode the input clock signal should be coupled by means of a capacitor of 100 pF (Cc in Figure 7, drawing a), with an
amplitude of at least 200 mVrms. X2 pin in this configuration can be left not connected. If slave mode is selected, F osc signal of
50-50 duty cycle can range from 1 MHz to 50 MHz.
External components and models used in oscillation mode are shown in Figure 7, drawings b and c, and in Table 6. Since the
feedback resistance is integrated on chip, only a crystal and the capacitances C X1 and C X2 need to be connected externally in
case of fundamental mode oscillation (the fundamental frequency is represented by L, C L and R S ). Capacitance Cp in Figure 7,
drawing c, represents the parallel package capacitance and should not be larger than 7 pF. Parameters F C , C L , R S and C P are
supplied by the crystal manufacturer.
Choosing an oscillation mode as an on-board oscillator mode of operation limits F osc clock selection to 1 MHz to 30 MHz.
LPC2131/2132/2138
X1 X2
C C
Clock
Figure 7: Oscillator modes and models: a) slave mode of operation, b) oscillation mode of operation,
c) external crystal model used for C X1/X2 evaluation
Table 6: Recommended values for C X1/X2 in oscillation mode (crystal and external components parameters)
Fundamental Oscillation
Frequency F C
1 - 5 MHz
LPC2131/2132/2138
C X1
X1 X2
Xtal
a) b) c)
Crystal Load
Capacitance C L
Max. Crystal Series
Resistance R S
External Load
Capacitors C X1 , C X2
10 pF n.a. n.a.
20 pF n.a. n.a.
30 pF < 300 : 58 pF, 58 pF
System Control Block 31 November 22, 2004
C X2
L
C L
R S
C P