Linearizing an LED for 800 pwm steps'?

[HankMcSpank]

Hi Darrel,

thanks for he info...re the ext modifier, your example in the thread you linked to shows how to store one value in one location, but how would I store many follow on values....do I have to have a huge amount of similar associated lines

GOTO OverData             ; Make sure data doesn't try to execute
ASM
DataTable
    DW  1234h, 2178h     ; etc.
endasm
OverData:

So for example, let's say I want to store eight words, starting at location 2049 of codespace (of my pic12f1840), for example the following values....

256,257,258,259,260,261,262,263,264


what would that look like?

Once I get the overall vibe, then I'm rocking...... :-)

[Darrel Taylor]

There's no need to put it at a specific location (2049).
It's just in-line data that can be anywhere in code space.
The DataTable label let's the compiler know where it is.

The example shows putting two values in two locations.
Just continue putting more data in as needed. You can use multiple lines,

    DW 256,257,258,259,260,261,262,263,264
    DW 268,270 ... etc.

And rember that the READCODE has to be AFTER the DataTable.
Another reason not to put it at a specific address.

[Ioannis]

Do you have any freedom to make hardware changes?

PWM controlling the current of the LED will give the best results.

So you could use a PWM output to drive a programmable constant current which in turn will drive the LED.

It sounds complicated but is not that much. Look at the schematic.


PWM output is filtered and drives the transistor that operates as a constant current driver. Current is controlled by R2 and should be selected to satisfy max LED current.

R1 and C1 be 5-10 times the PWM frequency.

Ioannis

[HankMcSpank]

Darrel,

That works a treat...many thanks :-)

A monster 1024 value table .....& as compact as Mr Compact's very compact car ....which in turn has been compacted & stored in a very compact gap between two houses.

DataWord  VAR  WORD
counter1    VAR  WORD
DataTable CON  EXT
'-----[The DATA table]--------------------------------------------------------
GOTO OverData             ; Make sure data doesn't try to execute
ASM
DataTable
    DW 0,0,0,0,1,1,1,1,1,1,1,2,2,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,4,4,4,5,5,5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,8,9,9,9,9,9,9,10,10,10,10,10,10,10,11,11
    DW 11,11,11,11,12,12,12,12,12,12,13,13,13,13,13,13,13,14,14,14,14,14,14,15,15,15,15,15,15,16,16,16,16,16,16,16,17,17,17,17,17,17,18,18,18,18,18,18,19,19,19,19,19
    DW 19,20,20,20,20,20,20,21,21,21,21,21,21,22,22,22,22,22,22,23,23,23,23,23,23,24,24,24,24,24,24,25,25,25,25,25,25,26,26,26,26,26,27,27,27,27,27,27,28,28,28,28,28
    DW 28,29,29,29,29,29,29,30,30,30,30,30,31,31,31,31,31,31,32,32,32,32,32,33,33,33,33,33,33,34,34,34,34,34,34,35,35,35,35,35,36,36,36,36,36,37,37,37,37,37,37,38,38
    DW 38,38,38,39,39,39,39,39,40,40,40,40,40,40,41,41,41,41,41,42,42,42,42
    DW 42,43,43,43,43,43,44,44,44,44,44,44,45,45,45,45,45,46,46,46,46,46,47,47,47,47,47,48,48,48,48,48,49,49,49,49,49,50,50,50,50,50,51,51,51,51
    DW 51,52,52,52,52,53,53,53,53,53,54,54,54,54,54,55,55,55,55,55,56,56,56,56,56,57,57,57,57,58,58,58,58,58,59,59,59,59,59,60,60,60,60,61,61,61,61
    DW 61,62,62,62,62,63,63,63,63,63,64,64,64,64,65,65,65,65,65,66,66,66,66,67,67,67,67,67,68,68,68,68,69,69,69,69,70,70,70,70,70,71,71,71,71,72,72
    DW 72,72,73,73,73,73,74,74,74,74,74,75,75,75,75,76,76,76,76,77,77,77,77,78,78,78,78,79,79,79,79,80,80,80,80,81,81,81,81,82,82,82,82,83,83,83,83
    DW 84,84,84,84,85,85,85,85,86,86,86,86,87,87,87,87,88,88,88,89,89,89,89,90,90,90,90,91,91,91,91,92,92,92,93,93,93,93,94,94,94,94,95,95,95,96,96
    DW 96,96,97,97,97,97,98,98,98,99,99,99,99,100,100,100,101,101,101,101,102
    DW 102,102,103,103,103,103,104,104,104,105,105,105,105,106,106,106,107,107,107,108,108,108,108,109,109,109,110,110,110,111,111,111,111,112,112
    DW 112,113,113,113,114,114,114,115,115,115,116,116,116,116,117,117,117,118,118,118,119,119,119,120,120,120,121,121,121,122,122,122,123,123,123,124
    DW 124,124,125,125,125,126,126,126,127,127,127,128,128,128,129,129,129,130,130,130,131,131,132,132,132,133,133,133,134,134,134,135,135,135,136,136
    DW 137,137,137,138,138,138,139,139,139,140,140,141,141,141,142,142,142,143,143,144,144,144,145,145,146,146,146,147,147,147,148,148,149,149,149,150
    DW 150,151,151,151,152,152,153,153,153,154,154,155,155,156,156,156,157,157,158,158,158,159,159,160,160,161,161,161,162,162,163,163,164,164,165,165
    DW 165,166,166,167,167,168,168,169,169,169,170,170,171,171,172,172,173,173,174,174,175,175,175,176,176,177,177,178,178,179,179,180,180,181,181,182
    DW 182,183,183,184,184,185,185,186,186,187,187,188,188,189,189,190,190,191,191,192,193,193,194,194,195,195,196,196,197,197,198,198,199,200,200,201, 201,202,202,203
    DW 204,204,205,205,206,206,207,208,208,209,209,210,211,211,212,212,213,214,214,215,215,216,217,217,218,219,219,220,221,221,222,222,223,224,224,225,226,226,227,228
    DW 228,229,230,230,231,232,233,233,234,235,235,236,237,238,238,239,240,240,241,242,243,243,244,245,246,246,247,248,249,250,250,251,252,253,253,254,255,256,257,258
    DW 258,259,260,261,262,263,263,264,265,266,267,268,269,270,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295
    DW 296,297,298,299,300,301,303,304,305,306,307,308,309,311,312,313,314,315,317,318,319,320,322,323,324,326,327,328,329,331,332,334,335,336,338,339,341,342,344,345
    DW 347,348,350,351,353,354,356,358,359,361,362,364,366,368,369,371,373,375,377,378,380,382,384,386,388,390,392,394,396,398,401,403,405,407,410,412,414,417,419,422
    DW 424,427,429,432,435,437,440,443,446,449,452,455,458,462,465,468,472,475,479,483,487,490,495,499,503,507,512,517,521,526,532,537,543,548,554,561,567,574,581,589
    DW 597,605,614,624,634,645,657,670,684,699,717,736,759,786,819
    DW 862,922,1023
endasm
Overdata:   ' this is a jump point to make sure the above data table isn't executed
increment_loop:
if counter1 < 1023 then
COUNTER1 =COUNTER1+1
ReadCODE  (DataTable + COUNTER1), DataWord
duty = dataword
gosub change_pwm
pause 1
GOTO increment_loop
endif
decrement_loop:
if counter1 > 0 then
COUNTER1 =COUNTER1-1
ReadCODE  (DataTable + COUNTER1), DataWord
duty = dataword
gosub change_pwm
pause 1
GOTO decrement_loop
endif
GOTO increment_loop
change_pwm:
CCP1CON.4 = Duty.0       'Bit 0
CCP1CON.5 = Duty.1       'Bit 1
CCPR1L = Duty >> 2       'Bit 2-7
return

I've now get a smoothtastic fading up/down LED. (there did appear to be one gotcha...being of the lazy ilk, I wanted each DW line to be 256 values long - nope, max allowed seems to be about 65-70 bytes - I can't be exact but it is in or around that value ....so lots of DW lines needed)

Ioannis ...that sort of what I'm doing - but rather than a tranny I'm using a mosfet (no filtering either...since the end result is visual, and the eye is only good for about 25Hz ....my pwm is about 16khz...so I didn't see the need?)

[Ioannis]

There is no difference (significant at least) whether you use bipolar or mosfet since the transistor will work in linear region. So, since it works linearly, cap will make calculation easier, because you will have to deal with dc voltages in base of the transistor in regard with the ground.


So, after PWM fltering the resulting dc voltage is the sum of Vbe (~0,6) and V(R1). V(R1) will define the current through the LED.

Have not tested in comparisson with the table but seems easier to control.

Ioannis

[Mike, K8LH]

Hi Hank,

Have you tried using fewer discrete brightness level steps spread across the much larger pwm width? For example, I get very smooth fades using between 64 and 100 gamma corrected brightness level steps spread across a much larger 512 pwm steps.

You can also "bend" the duty cycle curve to help match the characteristics of the LEDs.

Regards, Mike

[HankMcSpank]

Hi Mike,

Thnaks for the input...no I've not tried fewer brightness steps - I figured my problem was an insufficient number of brightness steps ......especially at the low end of the brightness scale, where for example the first 20 steps at 256 pwm levels sees quite a swing in the perceived LED brightness.....this is why I whacked it right up to 1024 brightness levels....which is much better from a less 'steppy' LED response perspective , but I'm still not happy with the overall fade curve, so your formula is of high interest! (there's aplethora of curves out there...and it's difficult to decide which one is best for this application!)

I've copied your formula into an excel spreadsheet for LUT of 1024 (well, I have the code space so may as well use it!) & 10 bit PWM.....the curve looks more like what I'm aiming for (if you look at my earlier curve further up the thread, the last part of the curve was almost vertical - this is too extreme)....I see yours is a little more gradual/linear towards the upper/final part of the plot ....and I shall give this a try tomorrow...many thanks.


What's with the 0.3 at the end of the formula ...where did that come from?