LPC2458FET180,551 Datasheet LPC2458FET180,551 | P70-P72

Product data sheet Rev. 4.1 — 15 October 2013 70 of 81

NXP Semiconductors

LPC2458

Single-chip 16-bit/32-bit micro

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

(Figure 26

), 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 XTAL2 pin in this configuration can be left unconnected.

External components and models used in oscillation mode are shown in Figure 27

and in

Table 20

and Table 21. Since the feedback resistance is integrated on chip, only a crystal

and the capacitances C

and C

need to be connected externally in case of

fundamental mode oscillation (the fundamental frequency is represented by L, C

and

). Capacitance C

in Figure 27 represents the parallel package capacitance and should

not be larger than 7 pF. Parameters F

OSC

, C

, R

and C

are supplied by the crystal

manufacturer.

Fig 27. Oscillator modes and models: oscillation mode of operation and external crystal

model used for C

evaluation

Table 20. Recommended values for C

in oscillation mode (crystal and external

components parameters): low frequency mode

Fundamental oscillation

frequency F

OSC

Crystal load

capacitance C

Maximum crystal

series resistance R

External load

capacitors C

1 MHz to 5 MHz 10 pF < 300  18 pF, 18 pF

20 pF < 300  39 pF, 39 pF

30 pF < 300  57 pF, 57 pF

5 MHz to 10 MHz 10 pF < 300  18 pF, 18 pF

20 pF < 200  39 pF, 39 pF

30 pF < 100  57 pF, 57 pF

10 MHz to 15 MHz 10 pF < 160  18 pF, 18 pF

20 pF < 60  39 pF, 39 pF

15 MHz to 20 MHz 10 pF < 80  18 pF, 18 pF

002aag469

LPC2xxx

XTAL1 XTAL2

XTAL

Product data sheet Rev. 4.1 — 15 October 2013 71 of 81

NXP Semiconductors

LPC2458

Single-chip 16-bit/32-bit micro

14.3 RTC 32 kHz oscillator component selection

The RTC external oscillator circuit is shown in Figure 28. Since the feedback resistance is

integrated on chip, only a crystal, the capacitances C

and C

need to be connected

externally to the microcontroller.

Table 22

gives the crystal parameters that should be used. C

is the typical load

capacitance of the crystal and is usually specified by the crystal manufacturer. The actual

influences oscillation frequency. When using a crystal that is manufactured for a

different load capacitance, the circuit will oscillate at a slightly different frequency

(depending on the quality of the crystal) compared to the specified one. Therefore for an

accurate time reference it is advised to use the load capacitors as specified in Table 22

that belong to a specific C

. The value of external capacitances C

and C

specified in

this table are calculated from the internal parasitic capacitances and the C

. Parasitics

from PCB and package are not taken into account.

Table 21. Recommended values for C

in oscillation mode (crystal and external

components parameters): high frequency mode

Fundamental oscillation

frequency F

OSC

Crystal load

capacitance C

Maximum crystal

series resistance R

External load

capacitors C

CX2

15 MHz to 20 MHz 10 pF < 180  18 pF, 18 pF

20 pF < 100  39 pF, 39 pF

20 MHz to 25 MHz 10 pF < 160  18 pF, 18 pF

20 pF < 80  39 pF, 39 pF

Fig 28. RTC oscillator modes and models: oscillation mode of operation and external

crystal model used for C

evaluation

Table 22. Recommended values for the RTC external 32 kHz oscillator C

components

Crystal load capacitance

Maximum crystal series

resistance R

External load capacitors C

11 pF < 100 k 18 pF, 18 pF

13 pF < 100 k 22 pF, 22 pF

15 pF < 100 k 27 pF, 27 pF

002aaf495

LPC2xxx

RTCX1 RTCX2

32 kHz XTAL

Product data sheet Rev. 4.1 — 15 October 2013 72 of 81

NXP Semiconductors

LPC2458

Single-chip 16-bit/32-bit micro

14.4 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines

The crystal should be connected on the PCB as close as possible to the oscillator input

and output pins of the chip. Take care that the load capacitors C

, C

, and C

in case of

third overtone crystal usage have a common ground plane. The external components

must also be connected to the ground plain. Loops must be made as small as possible in

order to keep the noise coupled in via the PCB as small as possible. Also parasitics

should stay as small as possible. Values of C

and C

should be chosen smaller

accordingly to the increase in parasitics of the PCB layout.

14.5 Standard I/O pin configuration

Figure 29 shows the possible pin modes for standard I/O pins with analog input function:

• Digital output driver

• Digital input: Pull-up enabled/disabled

• Digital input: Pull-down enabled/disabled

• Analog input (for ADC input channels)

The default configuration for standard I/O pins is input with pull-up enabled. The weak

MOS devices provide a drive capability equivalent to pull-up and pull-down resistors.

Fig 29. Standard I/O pin configuration with analog input

ESD

weak

pull-up

weak

pull-down

output enable

output

pull-up enable

pull-down enable

data input

analog input

select analog input

002aaf496

pin configured

as digital output

driver

pin configured

as digital input

pin configured

as analog input

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LPC2458FET180,551

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ARM Microcontrollers - MCU ARM7 512KF/USBH/ENET

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