PWM (ECCP) - piezo-sounder directly connected to half-bridge on PIC

Hi there,

I'm finishing my cooking-timer project and first time tried the PWM (ECCP) half-bridge functionality (when pin P1A is HIGH, P1B is LOW, next cycle inverts and so forth) on my PIC 16f690.

The piezo-sounder I have works best (= is the loudest) @ 4kHz.

The idea (not new at all) is to augment my piezo-sounder's loudness by inverting its power polarity @ 4kHz (kind of 10V peak-to-peak). As a matter of fact, it works very well :)

Since my piezo-sounder draws not more than 1,5mA, I have connected it directly to the respective PIC's pins P1A & P1B.

Is this way of doing okay or am I going to have a surprise in a certain period of time?

Hello Roger,

I have done this many times ( because it works very well on getting a higher sound with 2 pins inverted ) and never had any problems.
The only difference is that I use a small resistor ( 100 ohms ) between the pin and the piezo ( one for each pin ).
The frequency depends on the piezo so it's best to test to see which is the optimal frequency ( as you did ).

Regards
Rui

Quote:

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Originally Posted by flotulopex

augment ... piezo-sounder's loudness by inverting its power polarity @ 4kHz (kind of 10V peak-to-peak).

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Please, can you post the code for this particular thing ? Thanks !

That's the code, well... it's just THE part of the cooking-timer code to make that "half-bridge" PWM work:

Code:

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'-------------------------------------------------------------------------------
' PIC 16F690 Fuses (MPASM)
@ __config _FCMEN_OFF &_IESO_OFF &_CPD_OFF &_WDT_OFF &_HS_OSC &_BOR_OFF &_CP_OFF &_PWRTE_OFF &_MCLRE_OFF

'-------------------------------------------------------------------------------
' Registers  76543210
OPTION_REG = %10000110 ' Pull-Ups OFF, Prescaler TMR0 1:128
ANSEL      = %00000000 ' Select analog inputs Channels 0 to 7
ANSELH    = %00000001 ' Select analog inputs Channels 8 to 11
WPUA      = %00000000 ' Select weak pull-ups
WPUB      = %00000000 ' Select weak pull-ups
ADCON0    = %10000000 ' AD Module
ADCON1    = %00000000 ' AD control register
CM1CON0    = %00000000 ' Comparator1 Module
CM2CON0    = %00000000 ' Comparator2 Module
INTCON    = %10100000 ' INTerrupts CONtrol; GIE=1, T0IE=1
TRISA      = %00000000 ' Select Input/Output (0 to 5)
PORTA      = %00000000 ' Set High/Low (0 to 5)
TRISB      = %00110000 ' Select Input/Output (4 to 7)
PORTB      = %00000000 ' Set High/Low (4 to 7)
TRISC      = %01000000 ' Select Input/Output (0 to 7)
PORTC      = %00000000 ' Set High/Low (0 to 7)

' HPWM registers
T2CON      = %00000101 ' Timer2 = ON (bit2), prescaler (bits1:0) 00=1, 01=4, 1x=16
CCP1CON    = %10001100 ' Select PWM Mode (%00001100 = PWM module ON)
CCPR1L    = 62        ' Dutycycle set to 50% (= percentage of PR2)
PR2        = 124      ' Freq 4kHz

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Just as a precaution I would recommend to have the outputs at the same logic level when you do not want the sounder work. Thats because piezo do not like DC voltage.

Ioannis

Thanks Mr.Flotulopex and Mr.Ioannis !
It's verry usefull.

Quote:

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...I would recommend to have the outputs at the same logic level...

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When the souder is not used, I set CCP1CON register to 0.

Is this okay?

....hum, sorry for my silly question :o

Just measured the ports and they need to be reset.

Thanks for noticing Ioannis ;)

As you observed Roger, you must be in luck to have both ports at the same level. They will be in a random state.

Ioannis