LPC1111 datasheet(48/59 Pages) NXP

48 / 59 page

LPC1111_12_13_14_1

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Product data sheet

Rev. 01 — 16 April 2010

48 of 59

NXP Semiconductors

LPC1111/12/13/14

32-bit ARM Cortex-M0 microcontroller

11. Application information

11.1 ADC usage notes

The following guidelines show how to increase the performance of the ADC in a noisy

environment beyond the ADC specifications listed in Table 8:

• The ADC input trace must be short and as close as possible to the LPC1111/12/13/14

chip.

• The ADC input traces must be shielded from fast switching digital signals and noisy

power supply lines.

• Because the ADC and the digital core share the same power supply, the power supply

line must be adequately filtered.

• To improve the ADC performance in a very noisy environment, put the device in Sleep

mode during the ADC conversion.

11.2 XTAL input

The input voltage to the on-chip oscillators is limited to 1.8 V. If the oscillator is driven by a

clock in slave mode, it is recommended that the input be coupled through a capacitor with

Ci = 100 pF. To limit the input voltage to the specified range, choose an additional

capacitor to ground Cg which attenuates the input voltage by a factor Ci/(Ci + Cg). In slave

mode, a minimum of 200 mV(RMS) is needed.

In slave mode the input clock signal should be coupled by means of a capacitor of 100 pF

(Figure 24), with an amplitude between 200 mV(RMS) and 1000 mV(RMS). This

corresponds to a square wave signal with a signal swing of between 280 mV and 1.4 V.

The XTALOUT pin in this configuration can be left unconnected.

External components and models used in oscillation mode are shown in Figure 25 and in

Table 17 and Table 18. Since the feedback resistance is integrated on chip, only a crystal

and the capacitances CX1 and CX2 need to be connected externally in case of

fundamental mode oscillation (the fundamental frequency is represented by L, CL and

RS). Capacitance CP in Figure 25 represents the parallel package capacitance and should

not be larger than 7 pF. Parameters FOSC, CL, RS and CP are supplied by the crystal

manufacturer (see Table 17).

Fig 24. Slave mode operation of the on-chip oscillator

LPC1xxx

XTALIN

100 pF

002aae788

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