PIC Programming for Ultrasonic Sensor Help.

Note - assumes a 20MHz crystal, which is 5MHz timer clock
// A 1:4 prescaler is used to give a 1.25MHz timer count (0.8uS per tick)

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brightjoey said:

I'm having trouble understanding this particular phrase in this Ultrasonic Sensor PIC Programming guide pdf file in page 8 of 12.
I assume it means the oscillator that is inserted in the PIC. But I still don't get "crystal" and the whole phrase. ><

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ericgibbs said:

Hi,
The PIC divides the crystal frequency of 20mHz by 4 to give a 5mHz internal clock for the PIC.

A prescaler which is internal to the PIC can be programmed to divide by certain ratio's.
A 1:4 prescale will reduce the internal clock for use with say an internal Timer by 4, the system clock will still be 5mHz, but the Timer clock will be 1.25mHz.

A crystal or xtal is an external device that generates in conjunction with the PIC a 20mHz frequency, in this case.

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range = get_srf04(); // get range from srf04 (round trip flight
time in 0.8uS units)
cursor(24); // sets cursor to 2nd row of LCD03
sprintf(s,"Range = %dcm ", range/72); // convert to cm
print(s); // send it to the LCD03
cursor(44); // sets cursor to 3rd row of LCD03
sprintf(s,"Range = %dinch ", range/185); // convert to inches
print(s); // send it to the LCD03
TMR1H = 0; // 52mS delay - this is so that the
SRF04 ranging is not too rapid
TMR1L = 0; // and the previous pulse has faded
away before we start the next one
T1CON = 0x21; // 1:4 prescale and running
TMR1IF = 0;
while(!TMR1IF); // wait for delay times
TMR1ON = 0; // stop timer

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brightjoey said:

In the same datasheet in page 9 of 12. In the bolded fonts What does it mean when the range is divided into 72 and 185? The notes say its to convert to cm and inches respectively but I still don't get it?

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ericgibbs said:

hi,

I cannot open your pdf,,???

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brightjoey said:

It works when I downloaded it though. Here's the online link.
https://www.electro-tech-online.com/custompdfs/2010/11/Attachmentphpname41365-1.pdf

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brightjoey said:

i don't understand why 72 and 185 was need to convert to cms or inches？
is this no. is specific?


thanz for your co-operation.

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range = get_srf04(); // get range from srf04 (round trip flight
time in 0.8uS units)
cursor(24); // sets cursor to 2nd row of LCD03
sprintf(s,"Range = %dcm ", range/72); // convert to cm
print(s); // send it to the LCD03
cursor(44); // sets cursor to 3rd row of LCD03
sprintf(s,"Range = %dinch ", range/185); // convert to inches
print(s); // send it to the LCD03
TMR1H = 0; // 52mS delay - this is so that the
SRF04 ranging is not too rapid
TMR1L = 0; // and the previous pulse has faded
away before we start the next one
T1CON = 0x21; // 1:4 prescale and running
TMR1IF = 0;
while(!TMR1IF); // wait for delay times
TMR1ON = 0; // stop timer

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brightjoey said:

In the bolded text, how did we get the 52ms delay?

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brightjoey said:

Oo, is the 52ms set as default? Can we change the delay time?
Examples provided would be really helpful.

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brightjoey said:

Does the crystal affect the value of the delay then?
And for example if I want it to have 100ms delay do I set "TMR1H = 100ms" instead of 0?

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