On Fri, 21 Nov 2003 14:26:00 +0100, you wrote:

>New to PIC's and not having a degree in electronics, I have a question:
>
>I want to drive an ultrasound transmitter (pi=EBzo, 40 kHz) directly =
from picpins. I tested this using the PWM on a 877A. The transmitter is =
connected between the pwm-output and Vdd. This produces a very good =
ultrasound sinewave.
>
>Next, to increase the transmitter's output, I connected the transmitter =
between RD0 and RD1 on a 18F452 running at 40Mhz and wrote some code to =
generate a 40kHz block. I toggle both pic-pins in a single cycle in an =
attempt to create a pure wave as much as possible. This means that one =
pin goes from sinking to sourcing and the other pin going from sourcing =
to sinking in a single 100ns cycle. This did not work a I expected. After=
 some more experimenting using the scope, I came with the following =
explanation:
>
>What I am doing asks to much from the pins as pin switching-times for =
sourcing to sinking is not equal to the time from sinking to sourcing. =
Furthermore, the timings on the pins are not necessarily equal on both =
pins (probably they are not). This results in effectivily shorting Vdd to=
 Vss for a very brief moment through both pin-drivers.
>
>Is this assumption correct? What timing should I use to do the toggling =
safely? Is seperating the toggle into two instructions enough?

This is probably not working as the capacitance of the transducer is =
causing read-modify-write
problems, i.e. when you write the second pin, the first has not reached =
its intended state, so the
wrong value is read and written back.

A simple solution is to switch bith pins simultaneously in one =
instruction, either :=20

 movlw 1         ; setup
 movwf portd
loop
 movlw 3
 xorwf portd ; this will invert both pins


or =20

loop
 movlw 1
 movwf portd
<delay>  =20
 movlw 2
 movwf portd


The former method has the advantage that it is easier to guarantee a 50% =
duty cycle, which can be
important for some types of piezo transducer

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