abstract.txt

PHILIPS LPC900 PWM CRTL 时间可调

LPC932 EXAMPLE:	CREATING A PWM SIGNAL USING A TIMER IN MODE 6
by Christian Kulig, Philips Semiconductor 12/06/02

INTRODUCTION:
The LPC932 can be used to create a Pulse Width Modulated PWM signal. 
That's an analog signal, with only 2 discrete levels, for example 0V 
and 5V and a  constant period. The current value of this signal at a 
certain point of time is proportional to its Duty Cycle. That's the 
High Time during one period divided by the period. It can also be 
calculated as the average value during a particular period. That 
means after low pass filtering, (e.g. RC circuit) the signal becomes 
analog, with an actual value controlled by the microcontroller. The 
PWM functionality enables the LPC932 to control for example the speed 
of DC motors or the brightness of electric lighting.

DESCRIPTION:
The general purpose timer T1 of the LPC932 is set up to PWM Mode 6. 
In this mode it becomes a  8 bit up-counter with a special 8 bit 
reload on overflow. The reload values are alternating and represent 
the High Time and one period minus the High Time (=Low Time) of the 
PWM signal. On each overflow there can be an interrupt or a toggle of 
a certain port pin. 

The assigned port pin of T1 is P0.7.

The example alternately increases and decreases the Duty Cycle of the 
PWM signal on port pin P0.7 between 0% and 100%.

Please note, that on the Keil MCB900 Board this port pin is not 
connected to any of the LEDs, nevertheless the PWM can be displayed 
with a oscilloscope.