PROTON Plus PICBASIC COMPILER

Ultrasonic Range Finding.

Ready made units

If you don’t have the resources or the time to build the ultrasonic range finder from scratch, then there are a couple of alternatives. The people at Daventech have created two ultrasonic range finding modules that are very easy to interface with, and produce surprisingly good results. The SRF04 is the more basic of the two modules and uses a simple interface method but requires more code in order to obtain useful results. The SRF08 on the other hand, is packed full of features that will keep any twiddler happy for hours. It uses a more involved interface method but requires very little code to product remarkably accurate results.

Interfacing with the SRF04.

The SRF04 ultrasonic range finder is a small PCB measuring only 43mm by 20mm. It has two transducers attached, one for transmit and one for receive. The illustrations below show the front and rear views of the SRF04.

Connections to the SRF04 are via four of its five PCB pads shown below.

As far as I can tell, there is no polarity protection or voltage regulation on the PCB, so the SRF04 must be supplied with no more than 5 Volts, and the correct polarity must be observed otherwise damage ‘will’ occur.

The TRIGGER pin causes the SRF04 to take a range reading when it is cycled high to low, and must be in the high state for no less than 10us.

The ECHO pin will remain in the high state until an echo is detected. Measuring the length of time that the pin stays high will give us a measure of time in microseconds that we can convert into distance. In order to convert the returned pulse length into centimetres, we need to divide by 36, and to convert to inches we divide by 73.

The full listing for a program to control the SRF04 with the PROTON+ BASIC compiler is shown below. The TRIGGER pin should be connected to PORTB.1 of the PICmicro, and the ECHO pin should be connected to PORTB.0.

Include "PROTON_20.INC"             ' Use the PROTON board with a 20MHz xtal
       
Symbol TRIGGER  = PORTB.1           ' Define pin for Trigger pulse
Symbol ECHO  = PORTB.0              ' Define pin for Echo pulse
    
Dim RANGE as Word                   ' 16 bit variable for Range
           
Delayms 200                         ' Wait for PICmicro to stabilise
Cls                                 ' Clear the LCD
                 
While 1 = 1
  Pulsout TRIGGER,15,HIGH           ' Produce a 15us high to low pulse
  RANGE = Pulsin ECHO,1             ' Measures the range in uS
  Delayms 10                        ' 10ms recharge period after ranging completes
  RANGE = RANGE / 62                ' Use 62 for cm or 149 for inches
  Print at 1,1,"RANGE = ", Dec Range," cm  "
Wend

Notice that were using a 20MHz crystal. This is because the resolution of the PULSIN command is simply not good enough for accurate results if a lower frequency oscillator is used. The resolution of PULSIN with a 20MHz crystal is 2us, which is more than enough for good accuracy.

Because we have a 2 microsecond resolution, the division to convert into centimetres or inches has to be doubled. So to convert to centimetres, we divide the returning pulse length by 62, and to convert to inches, we divide the pulse length by 146.

It is possible to break the returned pulse length into fractions of a centimetre or inch, but this is asking a little too much from ultrasonic range finding in general, never mind the little SRF04. However, by utilising the Median filter subroutine discussed earlier, the results from the SRF04 can be made extremely stable, and very accurate.

There’s not a lot else I can say about this module that isn’t shown in the demonstration program above. It really is that simple to use!

One thing that I found really useful was the fact that on the Daventech web site www.robot-electronics.co.uk , you can find the circuit diagram of the SRF04. And with a little modification, the range finder programs discussed earlier will work perfectly with it.

OK, so it’s not actually built on the PROTON development board’s solderless area, but the breadboard used is identical.

For ethical reasons, I cannot show the circuit diagram here. And for the same reasons, the circuit should only be used for educational or personal use. The people at Daventech have put a lot of thought into the SRF04, so they deserve our support.

Interfacing with the SRF08.

The SRF08 range finding module is the big brother of the SRF04, and is quite a sophisticated piece of equipment thanks, in part, to the on-board PICmicro and some clever circuitry. Interfacing with the SRF08 is accomplished using the popular 2-wire I²C bus, first developed by Philips for use in domestic appliances. The I²C bus is one of my favourite interfacing methods, so I feel right at home with this little device.

The SRF08 has exactly the same dimensions as the SRF04 which is 43mm by 20mm, and both modules are remarkably similar until you look closer, upon which, you will notice a small CDS light sensor located between the two ultrasonic transducers. So the SRF08 is also capable of measuring light level.

The SRF08 module appears as a Slave device on the I²C bus, with the controlling hardware, the PICmicro in our case, being the Master. And because the I²C protocol calls for open drain outputs, it requires a pullup resistor on both the SDA (Data) and SCL (Clock) pins, as illustrated below.

Ensure that a voltage of no more than 5 Volts is supplied to the SRF08, and that its polarity is observed, or damage ‘will’ occur.

The MCLR pin is not used for standard interfacing and can be left unconnected, but it can also be used to reset the module by briefly pulling it to ground.

The documentation that comes with the SRF08 is reasonably comprehensive, and somewhat involved, therefore it would be rather pedantic to repeat it here. So the best thing to do is to show you some code samples that will interface the SRF08 to a PICmicro using the PROTON+ BASIC compiler.

Upon first using the SRF08, its default address on the I²C bus, commonly called its SLAVE ADDRESS, is set to $E0. So by sending a series of commands to the device with the slave address of $E0, both Light and Range can be ascertained, either in centimetres, inches of raw Time Of Flight data measured in microseconds (uS).