Post by thread:12c508 "random delay"

Hi Kev. >I need to generate a "random" delay between 0.5 and 10 second using 12c508 >int osc, prescaled at whatever! I think that you need an n-bit random number generator that would satisfy the number of steps in the interval. Here is an 8-bit example (sorry, no code). I came across it somewhere in TI's MSP430 application notes some time ago. rnd(n) = rnd(n-1)*mult+inc n - iteration number rnd(0)= 123 - seed number mult = 221 - multiplier inc = 54 - prime number (I modeled this in MATLAB. Looks ok) Note, that result may be greater than 8 bit. Just leave only lower 8 bits. This will generate pseudo-random sequence. Addition is not a problem. Multiplication too can be found in Microchip's apps. Bye.

Kev, Well... If you you really want to learn how to walk.. Let's see, how to put the idea in words (; PIC ASM - words ;) 1. Delay = 0.5 - 10 sec 2. The random number generator will output numbers 0 - 255. This makes 256 steps resolution of the delay. (10 - 0.5) / 256 = 0.037 sec. 3. If you run 12c508 at 4 Mhz than instruction cycle time = 4 / 4e6 = 1e-6 sec and one step equals 0.037 / 1e-6 = 37109 instructions. To generate this delay you can choose between timer with prescaler and nested loops. One step delay using nested loops: ;*************************************** MiniDelay macro Local L1, L2 movlw x ;1 movwf Reg1 ;2 L1 movlw y ;1 ;3 ;6+d1 ;9+2*d1... movwf Reg2 ;2 L2 decfsz Reg2, f ;3;6;9.. goto L2 ;4;7; decfsz Reg1, f ;3+d1 ;6+2*d1 goto L1 ;4+d1 ;7+2*d1 endm ;*************************************** Internal loop will generate delay1 = 1 + 3 * y (instruction cycles) The whole loop will consume delay = 1 + 3 * x + (x * delay1) = 1 + 4 * x + 3 * y * x Note, that zero x or y is the same as 256 when using decfsz. In our case delay = 37109. Find x and y that would best fit :). 4. The delay can be broken in two parts - one constant (0.5 sec) and another variable (0 - 9.5sec). A common routine for both of them can be used. Let's call it VarDelay ;*************************************** ;Input: w - number of 0.037 sec mini-delays VarDelay iorlw 0 ;check if w=0 btfsc STATUS, Z return ;exit if w=0 movwf Reg3 ;store input number L3 MiniDelay decfsz Reg3, f ;repeat if needed goto L3 return ;*************************************** 5. Now the most interesting - Pseudo Random Number Generator. There may be a better approach, but for the beginning it's okay. (The problem is it has 128 numbers regularity) ;*************************************** ;Random Number Generator ;rnd(n) = rnd(n-1) * 221 + 54 ;*************************************** Rand ;Straight code multiplication: ;x * 221 = x * (1 + 4 + 8 + 16 + 64 +128) ;In this case we need only lower byte of result, so the higher byte is ignored movf Rnd, w ;w = rnd0 rlf Rnd, f ;rnd = 2 * rnd0 (MSB discarded, LSB not cleared) ;I don't know if leaving LSB as it is will increase randomness - it needs testing, but ;you can always clear it by including BCF Rnd, 0 rlf Rnd, f ;rnd = 4 * rnd0 addwf Rnd, w ;w = rnd0 + 4 * rnd0 rlf Rnd, f ;rnd = 8 * rnd0 addwf Rnd, w ;w = rnd0 + 4 * rnd0 + 8 * rnd0 rlf Rnd, f ;rnd = 16 * rnd0 addwf Rnd, w ;w = rnd0 * (1 + 4 + 8 + 16) rlf Rnd, f ;rnd = 32 * rnd0 rlf Rnd, f ;rnd = 64 * rnd0 addwf Rnd, w ;w = rnd0 * (1 + 4 + 8 + 16 + 64) rlf Rnd, f ;rnd = 128 * rnd0 addwf Rnd, w ;w = rnd0 * (1 + 4 + 8 + 16 + 64 + 128) = rnd0 * 221 addlw 54 ;w = rnd0 * 221 + 54 movwf Rnd ;Store result return ;*************************************** Hoooh... I hope this message won't make you stumble. Good luck in putting it all together. -----Original Message----- From: Kev Howard <spam_OUTKHo9196475TakeThisOuTAOL.COM> To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU <PICLISTKILLspamMITVMA.MIT.EDU> Date: Thursday, October 21, 1999 9:36 AM Subject: Re: 12c508 "random delay" >Hello Nikolai > > Thank you for the reply, but I dont yet have the experience to write the >code. >Iam trying to run before I have learned to walk!! > >Cheers > Kev > >