Linearizing an LED for 800 pwm steps'?

[Jerson]

Hi Hank

Why 800 or even 1024 steps? Why not 256? You may need even lesser!!!
Yes the lookup table approach is appropriate. However, since the majority brighness changes happen in the bottom segment, you need a very tiny step size/value changes at those levels. The easiest way to create this table of PWM values is to take your favourite desktop programming language and write a small program that will print out the PWM values. I have been used to using QuickBasic from MS for such stuff.

j = 1
for i = 0 to 256 step j                ' keep widening the step size as we don't need high resolution at larger PWM values
    print log(i)*256;",";               ' print a value like this "val,"  and stay on the line
    j = j+1
next

This, of course, is not the best code, but, just a guideline to get you moving.

Regards

[HankMcSpank]

thanks ...i know how to do 256 steps....the problem is it's not enough resolution

I likely need 1024 steps (even a pwm value of 1 has the led quite visible).....at 256 steps a PWM value of 1 is too bright.

I can obviously just use 1024 steps and drive the LED that way ...but without some form of linearizing lookup/data table the LED doesn't fade up/down smoothly (all the brightness action is slanted towards the lower end of the PWM scale)

i've since worked out how to generate the curves I need....but what I still can't figure out is how to put this resulting lookup data into code space.

I read Melanie's "use code space as your playground"....but her syntax relates to PBP assembler....and I'm too clueless to map it to MPASM!

I tried pokecode....but it baulked when the data lookup values got above 255.

Since I can't use Lookup2 .....all I want to do is (for a starter!) get the following 512 lookup values into my program/codespace (my PIC has 7KB of program space ...my program is only about 1800 words...so should be sufficient space to cram list of 512 words in)....

0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,
50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,
96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,
131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,
165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,
199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,
233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,
267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,
301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,
335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,
370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,
405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,
441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,
477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511

Just to give a little more detail wrt the end goal - I'm wanting to take an AD reading & convert it to LED brightness ....I'm using 10 bit ADC...so my intention is to divide the AD result by two then use that AD value to reference the corresponding bit of 512 words in that list ....therefore if incoming AD reading is 760 (10 bit reading)....divide it by 2 to get a 9 bit reading of 380 .....now reference the for 380th entry in that sequence of values ...that'll be the PWM value I need to use.


I've been searching/reading all night & I'm as confused now as whe I started - is this even do-able?

[spcw1234]

Could you do something like this?

ADCvalue var word
x        var byte
duty     var byte

main:
adcin 0, ADCvalue
x=adcvalue/2

if x<256 then lookup x, [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,_
50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,_
96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,_
131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,_
165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,_
199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,_
233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255],duty

if x>255 then lookup x, [256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,_
301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,_
335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,_
370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,_
405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,_
441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,_
477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511],duty

hpwm 1, duty, 1000

goto main

[Jerson]

of course it is do-able. However, I don't think you'll be too happy with the linearity. I am currently working with a similar led lighting problem and know what you're talking about. You need very fine grain at the bottom of the curve to get the intensity variations you need. Shawn has shown an excellent way to split the table lookup into 2 parts.

[HankMcSpank]

Thanks guys, I'm pursung Shawn's method now, a couple of things I've spotted that might need adjustment...

if x>255 then lookup2 x-255, [256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,_
301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,_
335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,_
370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,_
405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,_
441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,_
477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511],duty

In other words, for the second table I think I need to use lookup2 ( as the data in the send group of lookups are words), also I need to take 256 off the index value ....else it's trying to lookup the 256th->512th values in the data list .....but the data list runs 0 thru 255.

Anyway, many thanks....I'll let you know how I get on!

[HankMcSpank]

Boo hiss - No workeee, man cries.

lookup2 only allows up to 85 values in the list. ...I then tried to group all the lookup data I need into blocks of 85 values....but I spontaneously combusted (which gave my wife a bit of a fright)

Any other ideas how to get creative ...I just need to reference at least 512 values in a list (actually, I'd ideally like 1024 values) for a modest 12f1840? (it's got 7kb of program space)

[spcw1234]

What if you keep all the values below 256 and do multiple lookup tables with some simple maths.

ex.

if X<256 then lookup X, [0,...255],duty
if X>255 and X<512 then lookup X-256, [0,...255],duty
if X>511 and X<768 then lookup X-512, [0,...255],duty
if X>767 then lookup X-768, [0,...255],duty

You will loose some resolution in the upper bits, but that is where it doesn't matter for this application.

[Darrel Taylor]

lookup2 only allows up to 85 values in the list. ...

That was increased to 1024 in PBP3.
Bug fixes and tweaks:

• Limit on LOOKUP2 item list increased to 1024 for Mid-Range and Enhanced Mid-Range architectures.
• ...

But each value in a LOOKUP2 requires 3 WORDs.
1024 values will use 3K from the 4K available on the 12F1840..

Any other ideas how to get creative ...I just need to reference at least 512 values in a list (actually, I'd ideally like 1024 values) for a modest 12f1840? (it's got 7kb of program space)

This method can store 1024 14-bit values in 1024 words of program space.
http://www.picbasic.co.uk/forum/show...php?t=3891#LAB