TheBackShed.com

Forum Index : Microcontroller and PC projects : Pulse Widths

Author

Message

vk4tec

Senior Member

Joined: 24/03/2012
Location: Australia
Posts: 239

Posted: 01:32pm 20 Jul 2013

Copy link to clipboard

Print this post

Hello

When I use a PIC micro in assembly code I can predict the width of an ON/OFF/ON based upon the crystal frequency.

Should the MaxiMite / Duinomite be any different ?

I would like to make a RADAR signal simulator

- Andrew -

Andrew Rich VK4TEC
www.tech-software.net

Geoffg

Guru

Joined: 06/06/2011
Location: Australia
Posts: 3272

Posted: 08:13pm 20 Jul 2013

Copy link to clipboard

Print this post

If you are talking about the PULSE command then its timing is derived from the crystal.

Geoff

Geoff Graham - http://geoffg.net

vk4tec

Senior Member

Joined: 24/03/2012
Location: Australia
Posts: 239

Posted: 08:24pm 20 Jul 2013

Copy link to clipboard

Print this post

I have found that 20 MHz xtal on my PIC gives me

200 ns steps

08 MHz = 500 ns
10 MHz = 400 ns
20 MHz = 200 ns
16 MHz = 250 ns
32 MHz = 125 ns

My signals are

1. 0.8 us wide
2. 8 and 21 us apart

20 MHz does this for me

4 x 200 ns = 0.8 us (check)
40 x 200 ns = 8 us (check)
105 x 200 ns = 21 us (check)Edited by vk4tec 2013-07-22

Andrew Rich VK4TEC
www.tech-software.net

Dylan
Regular Member

Joined: 17/06/2013
Location: Netherlands
Posts: 81

Posted: 02:58pm 23 Jul 2013

Copy link to clipboard

Print this post

Since obviously 20Mhz = 1/(50 ns), I take it your inner delay loop executes in 4 assembly cycles.

The PIC32MX has two clocks, one 8MHz and the other 32.768kHz.

I can't help but notice that 1000000/(0.8+8+0.8+21)s is 32680 Hz, and even if it is purely coincidental that your high PRF is so near to a timer clock, from my very limited* understanding of radar, the signal is coming in too fast to tell what position the objects are in, but that speed can be judged more accurately at short ranges (*wiki example is speed traps).

The advantage of using an actual programming language, even one as basic as ... Basic, is that you can let it do more of the solving of the problem for you. Unfortunately, timer interrupts proved less than ideal for bit-banging PWM for me, and would be even less useful to you.

Print this page

To reply to this topic, you need to log in.