Datasheet

Product data sheet Rev. 2 — 26 August 2011 23 of 61

NXP Semiconductors

LPC122x

32-bit ARM Cortex-M0 microcontroller

• Selectable time period from (T

cy(WDCLK)

 256  4) to (T

cy(WDCLK)

 2

 4) in

multiples of T

cy(WDCLK)

 4.

• The Watchdog Clock (WDCLK) source can be selected from the Internal RC oscillator

(IRC) or the Watchdog oscillator. This gives a wide range of potential timing choices of

Watchdog operation under different power reduction conditions. It also provides the

ability to run the WDT from an entirely internal source that is not dependent on an

external crystal and its associated components and wiring for increased reliability.

7.17 Real-time clock (RTC)

The RTC provides a basic alarm function or can be used as a long time base counter. The

RTC generates an interrupt after counting for a programmed number of cycles of the RTC

clock input.

7.17.1 Features

• Uses dedicated 32 kHz ultra low-power oscillator.

• Selectable clock inputs: RTC oscillator (1 Hz, delayed 1 Hz, or 1 kHz clock) or main

clock with programmable clock divider.

• 32-bit counter.

• Programmable 32-bit match/compare register.

• Software maskable interrupt when counter and compare registers are identical.

• Generates wake-up from Deep-sleep and Deep power-down modes.

7.18 Clocking and power control

7.18.1 Crystal oscillators

The LPC122x include four independent oscillators. These are the system oscillator, the

Internal RC oscillator (IRC), the RTC 32 kHz oscillator (for the RTC only), and the

Watchdog oscillator. Except for the RTC oscillator, each oscillator can be used for more

than one purpose as required in a particular application.

Following reset, the LPC122x will operate from the Internal RC oscillator until switched by

software. This allows systems to operate without any external crystal and the bootloader

code to operate at a known frequency.

See Figure 5

for an overview of the LPC122x clock generation.