Ultrasonic Generator with MCU

I want to build an ultrasonic generator (variable frequency if possible-if not at 40kHz) with the use of a 89c51 for use in ultrasonic cleaning tank. The reason for the selection of the specific MCU is that i have some experience in programming it with Assembly language in other projects.
My thought is to use one of the controller timers in order to produce the 40kHz continuous pulse. I am thinking to use Timer 2 in PWM mode. I am also thinking to use a D/A converter in order to convert the output of the MCU and then a combination of a CMOS NAND gate , 2 NPN tranzistors and 8 CMOS inverters in order to drive the signal with enough power to the transducer.

I want to ask if the way I am thinking is the proper one.
Secondly what type of MCU clock should I use in order to take the 40kHz signal. 12MHz will be enough?
If someone has other proposals I like to hear them.
Thanks

Drive the transducer in a differential mode as per the drawing https://obrazki.elektroda.pl/11_1334178539.png

This is how I was thinking to drive it...One more question. I am thinking of not using a timer. The way I am thinking is the following...
loop:setb Px.0 ;high pulse
delay ;delay for 12.5μsec
crl Px.0; low pulse
delay; delay for 12.5μsec
jmp loop
Will I need to connect a D/A converter in the Px.0 pin?
What output the converter will give me? 0-5 Vp-p (0 for the low pulse and 5 for the high pulse)?
Thanks for your reply.

You can generate 40kHz reasonably accurately with a 555! Which costs about $0.05 and no programming.
Anyway, regardless of the method (555 or microcontroller), I doubt you need a DAC. Just pass the square wave to the transducer. It most likely won't
respond to anything except the 40kHz component of the square wave. So the output will be a naturally (mechanically) filtered 40kHz output.
It is possibly re-inventing the wheel for no good purpose by trying to use a microcontroller and DAC.

I thought about the 555. The problem I thing is that probably I am not able to generate a variable frequency generator with 555 and that's why I am thinking to use a microcontroller (if I am wrong please tell me).
If I will use the 555 I will need and the differential driver in order the square wave to have more power?
Thanks

The time resolution of a standard x51 processor will be at best one instruction cycle (1 us). That's not particularly fine in terms of frequency, 4% step size, respectively 8% when keeping the duty cycle.

i think 40 Khz ultrasonic transducer need about 18 pk-pk voltage to transmit waves. but 7404N inverter operates at 5V. so how to get voltage of 18V for tx.

I think that's better to use an ultrasonic module, like this: http://iteadstudio.com/store/index.php?main_page=product_info&cPath=4&products_id=415.

The only thing that microcontroller will need to do is count de time between the pulses and calculate de distance.