
LPC2921_23_25_3
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NXP B.V. 2010. All rights reserved.
Product data sheet
Rev. 03 — 14 April 2010
36 of 84
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.