LPC2921 datasheet(36/83 Pages) NXP

36 / 83 page

LPC2921_23_25_1

Preliminary data sheet

Rev. 01 — 15 June 2009

36 of 83

NXP Semiconductors

LPC2921/2923/2925

ARM9 microcontroller with CAN, LIN, and USB

6.14.4.3

Clock description

The ADC modules are clocked from two different sources; CLK_MSCSS_ADCx_APB and

CLK_ADCx (x = 1 or 2), see Section 6.7.2. Note that each ADC has its own CLK_ADCx

and CLK_MSCSS_ADCx_APB branch clocks for power management. If an ADC is

unused both its CLK_MSCSS_ADCx_APB and CLK_ADCx can be switched off.

The frequency of all the CLK_MSCSS_ADCx_APB clocks is identical to

CLK_MSCSS_APB since they are derived from the same base clock

BASE_MSCSS_CLK. Likewise the frequency of all the CLK_ADCx clocks is identical

since they are derived from the same base clock BASE_ADC_CLK.

The register interface towards the system bus is clocked by CLK_MSCSS_ADCx_APB.

Control logic for the analog section of the ADC is clocked by CLK_ADCx, see also

Figure 6.

6.14.5 Pulse Width Modulator (PWM)

The MSCSS in the LPC2921/2923/2925 includes four PWM modules with the following

features.

• Six pulse-width modulated output signals

• Double edge features (rising and falling edges programmed individually)

• Optional interrupt generation on match (each edge)

• Different operation modes: continuous or run-once

• 16-bit PWM counter and 16-bit prescale counter allow a large range of PWM periods

• A protective mode (TRAP) holding the output in a software-controllable state and with

optional interrupt generation on a trap event

• Three capture registers and capture trigger pins with optional interrupt generation on

a capture event

• Interrupt generation on match event, capture event, PWM counter overﬂow or trap

event

• A burst mode mixing the external carrier signal with internally generated PWM

• Programmable sync-delay output to trigger other PWM modules (master/slave

behavior)

6.14.5.1

Functional description

The ability to provide ﬂexible waveforms allows PWM blocks to be used in multiple

applications; e.g. dimmer/lamp control and fan control. Pulse-width modulation is the

preferred method for regulating power since no additional heat is generated, and it is

energy-efﬁcient when compared with linear-regulating voltage control networks.

The PWM delivers the waveforms/pulses of the desired duty cycles and cycle periods. A

very basic application of these pulses can be in controlling the amount of power

transferred to a load. Since the duty cycle of the pulses can be controlled, the desired

amount of power can be transferred for a controlled duration. Two examples of such

applications are:

• Dimmer controller: The ﬂexibility of providing waves of a desired duty cycle and cycle

period allows the PWM to control the amount of power to be transferred to the load.

The PWM functions as a dimmer controller in this application.

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