PWM Confusing

[canavaroski90]

Hi CCS forums.
Nowadays I'm working on a gas detection project. I'm trying to change leds blinking frequency according to the value which I've get from ADC. For example, if ADC value is below then 5, then led's going to blink at 1Hz otherwise led's going to blink at 5Hz.

I've read lots of article about "PWM and low frequencies" and I saw that you can not use PWM hardware to generate too low frequencies (i.e 5Hz).

Mainly, I'm confusing about "POWER" and "FREQUENCY". Here's my question: If I want to change brightness level of led, I can use PWM. This is "POWER" line of PWM. However if I want to use flashing led in different speed, again, I can use PWM. And this is the "FREQUENCY" line of PWM. But how can I allocate two of them? How would I configure my program to allocate two of them?

Regards.
(Sorry for bad english )
_________________
Liberate your mind..

[FvM]

How about adding more confusion? You can even have three índependant parameters associated with a LED driver:
- a PWM duty cycle (or "POWER") to set a reduced intensity. The PWM frequency would be sufficient high to make the light appear continuous (> 100 Hz)
- an optional software blink frequency
- a blink duty cycle, ranging from short flash to symmetrical (50 %) blink

[Ttelmah]

Forget about 'power'....

The PWM, does one thing. Generates a rectangular waveform, allowing you to specify a rate and 'pulse width'. Hence 'Pulse width modulation'.

Power is an 'outcome'. If you connect the a pulse with a width that is 25% of it's repetition rate, to a solid state switch, this switch will be 'on' 25% of the time, in many cases this gives an effective 25% 'power'. However with a lot of drivers it may not be as simple as this.

Now, you don't really appear to want PWM. Unless you want to change the brightness of the LED, it is not the tool for the job. Much simpler to have a system 'tick' running at some frequency like 50Hz, and just a counter switching the LED. Set the counter to 10, and the LED will flash at 5Hz, set it to 50, and the LED will flash at 1Hz.
If you want to change the intensity of the LED in it's flashes, then use a PWM for this, turning it on/off as needed, instead of just switching the LED, and adjusting it's mark/space ratio to give the required brightness.

Best Wishes

[canavaroski90]

Thanks for replies. Now I'm much clear about PWM and I want to ask another question.

I'm getting lots of variable values from 10 bit (16f877/A @4Mhz) ADC and I want to make this project very sensitive to gas. Because of this I'm only working on little ADC values (max 50). I've prepared 5 "if" code block for this, ADC between 0/10,10/20,20/30,30/40 and 40/50.

I want to configure my program as this:

ADC Value -- LED Flashing Frequency
0-10 1 Hz
10-20 5 Hz
20-30 10 Hz
30-40 15 Hz
40-50 Always on


If I use "delay_ms" function to get these values, simply to get 1 Hz I'm using these codes;

[PCM programmer]

[canavaroski90]

Thanks for replies. I'm much more clear about pwm now.
_________________
Liberate your mind..