Smooth LED fading via PWM

[HenrikOlsson]

Hi Hank,
No, not really....

Defalt_Config DATA 255

Will store the value 255 at location 0 in EEPROM and give that location the label or name Default_Config. If you want a value to be stored at location 255 you need to use

DATA @255, 42

Or possibly (not clear to me how the label thing it works)
Default_Config DATA @255, 42[code]
and then when the user changes that value you do

userValue VAR BYTE
WRITE 255, userValue

or possibly (still not clear how that "label" works)

userValue VAR BYTE
WRITE Default_Config, userValue

Sorry if I'm confusing you...
/Henrik.

[HankMcSpank]

Ok Henrik...I've just used your 'read the EEprom serially out onto my PC screen' (rather than try to work how to look at the data with the pickit2 itself) - it worked a treat....and certainly alleviates some angst as to "I wonder what's what inside the Eeprom space)

And I was clearly wrong with respect to getting my user defined byte tucked away to slot 255, this is obviously what I needed...

WRITE 255, default_config_shadow

(definitely a case of RTFM!)

Anyway, I've learnt something from this little jaunt - I'm happy :-) (tks Henrik)

Edit: Oops - just seen your reply (was obviously typing mine while you were replying) ...anyway, the penny has dropped here!

[SteveB]

Hank,
Something to tuck away in your cranium while working with the EEPROM: It has a limited number of write cycles.

I can't remember the number of write cycles, but you will NEVER wear it out just by reprograming it. (EDIT: The 18F4620 is rated minimum 100K and typical 1M write cycles)

But you could wear it out if you repeatedly write a value during the operaitons of your code. So only make the WRITE when the user actually changes the vaule.

[HankMcSpank]

Thanks Steve ...fortunately, this is likely to be a one shot selectable option (ie set & forget) - I doubt it'll ever be updated more than a few times.

[Charles Linquis]

I have to admit that I haven't read all the posts, but I noticed one thing awhile "up". PLEASE don't drive a LED directly from a PIC pin! LEDs are not 'lamps'. LEDs ARE DIODES! It is CURRENT, not voltage that lights them. Since they are a diode, if you put a slowly rising voltage across them, they (at first) draw no current whatsoever. Then, when they get to the diode forward voltage (which is determined by the technology of the LED), all of the sudden, they light. So, you can have no light at all at 2.2V, and burn them up at 2.3V. If a LED with a forward voltage of 2.3V was connected to a 3V source, it wouldn't last long. So why do they work when you connect them directly to PIC pins? It is because the PIC pins have very small FETs driving them, and they have an "effective" resistance. That is, they cannot supply a huge amount of current. This current-limiting allows both the PIC and the LED to survive.
You have seen the LED flashlights that have nothing but white LEDs and 3 - AAA batteries. No resistor. The forward voltage of most white LEDs is between 3.1 and 4V (Wikipedia). The 3 batteries in series produce about 4.5V. It all works because batteries are not perfect power sources, they have an Equivalent Series Resistance (ESR) that acts like a resistor in series with the battery. This ESR allows things to work - kind of. But have you ever noticed that while you expected the LED flashlights to work forever - they don't? They fail because the makers don't put in a little extra resistance in the leads of each LED to balance the LEDs and to control the current.

Back to PICs: The PIC pins outputs are not really resistors. They may act somewhat like resistors, but they are not. The "resistance" of the pin can vary quite a bit over Vcc variations and from device to device and even pin-to-pin. As a result, the results can be unpredictable. Please put a resistor (at least) in series with your LED.

[HankMcSpank]

The PIC pins outputs are not really resistors. They may act somewhat like resistors, but they are not. The "resistance" of the pin can vary quite a bit over Vcc variations and from device to device and even pin-to-pin. As a result, the results can be unpredictable. Please put a resistor (at least) in series with your LED.

I agree they're not what we'd deem traditional resistors...but for supply of 3.268V to the (blue) LED, I'm seeing 0.337V dropped across the PIC pin - that equates to a series DC resistance in any language. I take your point about variations (though I'm not seeing it pin to pin). So with the PIC pin V drop, that only leaves about 2.95V for the blue LED (with a rated fwd voltage of about 3.2V) ...there's not much chance a blue LED will blow with just 2.95V across it (the current through it is fairly meagre at that voltage level). Fear not, I appreciate your input (& concern), but seriously...this isn't a production run - this is for my own use (& a couple of mates if my project-ette turns out to my liking) ...simply towards learning about PICs & pushing my own (small!) boundaries...I posted up my earlier findings for interest (wrt actual real world IO pin resistance) as opposed to evangelising to others about going 'resistorless'!

I do wonder though since 50 ohms is a surprisingly high figure, just how many factor the PIC's pin resistance in when calculating a LED resistor value? (I suspect not many) ...the resistance certainly can't be disregarded becuase certainly with relatively low value resistors normally in play for LEDs, the PIC pin's inherent resistance is gonna severely skew the desired end result.

[Charles Linquis]

I generally use 2N7002's for driving things like that. They are great, logic-level N-channel FETs that have a pretty low ON resistance. Cheap too.

[sayzer]

Only if using PWM on a PIC pin , you can directly drive a transistor base without a resistor.
That will save you one component (resistor) and also space on PCB.

If you use Mosfet with PWM, you can also directly drive the gate without resistor. But this time, you need to have a resistor between the gate and GND. This resistor is not needed if not using PWM.

[HankMcSpank]

I generally use 2N7002's for driving things like that. They are great, logic-level N-channel FETs that have a pretty low ON resistance. Cheap too.

Since I'm sinking current into the pin, I'm using a common (ie shared by all pins) P Channel Mosfet (with something like .2 Ohm 'on' resistance) & inverted PWM on the ECCP module ....it works well.

[Ioannis]

For any N-Channel small MosFet needs (up to 2A) have a look at the Si2308 too. It is working fine from 4.5 Volts with Rds of 0.192 and Id of 2.1 Amp.


And the best P-Channel Power Mosfet I have find maybe is the SUM110P04-05-E3. Usually the P-channel have high Rds and low Id specs.

Ioannis

[HankMcSpank]

actually, I was mistaken the RDS of my Mosfet is about 0.7 Ohms.

I bought these P channel Mosfets when CPC Farnell were clearing them out for about 5p each...

http://www.farnell.com/datasheets/468737.pdf

I think I bought about 100 ....I couldn't believe it when they arrived...every Mosfet was sealed individually in a 5cm plastic bag, & then inside the plastic bag, each MOSFET was then sealed in it's SMD wrapping! It takes me more time to get to the mosfet than to make my PCB!

[HankMcSpank]

Ok, I missing somethinh fundamental here...wondering if anyone can embarrass me!

I'm putting some data into eeprom...

data  0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, _
1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, _
2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4, _
4,4,5,5,5,5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8, _
8,8,8,8,9,9,9,9,10,10,10,10,10,11,11,11,11,12,12,12, _
13,13,13,13,14,14,14,15,15,15,16,16,16,17,17,18,18,18, _
19,19,20,20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28, _
29,29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,42, _
43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,62,63,64, _
66,67,69,70,72,74,75,77,79,80,82,84,86,88,90,91,94,96,98, _
100,102,104,107,109,111,114,116,119,122,124,127,130,133,136, _
139,142,145,148,151,155,158,161,165,169,172,176,180,184,188, _
192,196,201,205,210,214,219,224,229,234,239,244,250,252

note that last value 252....ok, when I read out the contents of eeprom...

Loc000: 000
Loc001: 001
Loc002: 001
Loc003: 001
Loc004: 001
Loc005: 001
Loc006: 001
Loc007: 001
Loc008: 001
Loc009: 001
Loc010: 001
Loc011: 001
Loc012: 001
Loc013: 001
Loc014: 001
Loc015: 001
Loc016: 001
Loc017: 001
Loc018: 001
Loc019: 001
Loc020: 001
Loc021: 001
Loc022: 001
Loc023: 001
Loc024: 001
Loc025: 001
Loc026: 001
Loc027: 001
Loc028: 001
Loc029: 001
Loc030: 001
Loc031: 002
Loc032: 002
Loc033: 002
Loc034: 002
Loc035: 002
Loc036: 002
Loc037: 002
Loc038: 002
Loc039: 002
Loc040: 002
Loc041: 002
Loc042: 002
Loc043: 002
Loc044: 002
Loc045: 002
Loc046: 002
Loc047: 002
Loc048: 002
Loc049: 002
Loc050: 003
Loc051: 003
Loc052: 003
Loc053: 003
Loc054: 003
Loc055: 003
Loc056: 003
Loc057: 003
Loc058: 003
Loc059: 003
Loc060: 003
Loc061: 003
Loc062: 003
Loc063: 004
Loc064: 004
Loc065: 004
Loc066: 004
Loc067: 004
Loc068: 004
Loc069: 004
Loc070: 004
Loc071: 004
Loc072: 004
Loc073: 004
Loc074: 005
Loc075: 005
Loc076: 005
Loc077: 005
Loc078: 005
Loc079: 005
Loc080: 005
Loc081: 005
Loc082: 006
Loc083: 006
Loc084: 006
Loc085: 006
Loc086: 006
Loc087: 006
Loc088: 006
Loc089: 007
Loc090: 007
Loc091: 007
Loc092: 007
Loc093: 007
Loc094: 007
Loc095: 008
Loc096: 008
Loc097: 008
Loc098: 008
Loc099: 008
Loc100: 008
Loc101: 009
Loc102: 009
Loc103: 009
Loc104: 009
Loc105: 010
Loc106: 010
Loc107: 010
Loc108: 010
Loc109: 010
Loc110: 011
Loc111: 011
Loc112: 011
Loc113: 011
Loc114: 012
Loc115: 012
Loc116: 012
Loc117: 013
Loc118: 013
Loc119: 013
Loc120: 013
Loc121: 014
Loc122: 014
Loc123: 014
Loc124: 015
Loc125: 015
Loc126: 015
Loc127: 016
Loc128: 016
Loc129: 016
Loc130: 017
Loc131: 017
Loc132: 018
Loc133: 018
Loc134: 018
Loc135: 019
Loc136: 019
Loc137: 020
Loc138: 020
Loc139: 020
Loc140: 021
Loc141: 021
Loc142: 022
Loc143: 022
Loc144: 023
Loc145: 023
Loc146: 024
Loc147: 024
Loc148: 025
Loc149: 026
Loc150: 026
Loc151: 027
Loc152: 027
Loc153: 028
Loc154: 029
Lo1155: 029
Loc156: 030
Loc157: 031
Loc158: 031
Loc159: 032
Loc160: 033
Loc161: 033
Loc162: 034
Loc163: 035
Loc164: 036
Loc165: 036
Loc166: 037
Loc167: 038
Loc168: 039
Loc169: 040
Loc170: 041
Loc171: 042
Loc172: 042
Loc173: 043
Loc174: 044
Loc175: 045
Loc176: 046
Loc177: 047
Loc178: 048
Loc179: 050
Loc180: 051
Loc181: 052
Loc182: 053
Loc183: 054
Loc184: 055
Loc185: 057
Loc186: 058
Loc187: 059
Loc188: 060
Loc189: 062
Loc190: 063
Loc191: 064
Loc192: 066
Loc193: 067
Loc194: 069
Loc195: 070
Loc196: 072
Loc197: 074
Loc198: 075
Loc199: 077
Loc200: 079
Loc201: 080
Loc202: 082
Loc203: 084
Loc204: 086
Loc205: 088
Loc206: 090
Loc207: 091
Loc208: 094
Loc209: 096
Loc210: 098
Loc211: 100
Loc212: 102
Loc213: 104
Loc214: 107
Loc215: 109
Loc216: 111
Loc217: 114
Loc218: 116
Loc219: 119
Loc220: 122
Loc221: 124
Loc222: 127
Loc223: 130
Loc224: 133
Loc225: 136
Loc226: 139
Loc227: 142
Loc228: 145
Loc229: 148
Loc230: 151
Loc231: 155
Loc232: 158
Loc233: 161
Loc234: 165
Loc235: 169
Loc236: 172
Loc237: 176
Loc238: 180
Loc239: 184
Loc240: 188
Loc241: 192
Loc242: 196
Loc243: 201
Loc244: 205
Loc245: 210
Loc246: 214
Loc247: 219
Loc248: 224
Loc249: 229
Loc250: 234
Loc251: 239
Loc252: 244
Loc253: 250
Loc254: 252
Loc255: 255

I get 252 as the 2nd to last number....with 255 being the final number (I'm guessing all 1s is a default value of unused eeprom slots?)

therefore there must be just 255 values in that table, but if I add another value on the end (the 256th slot) ...saya number of 253...

data  0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, _
1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, _
2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4, _
4,4,5,5,5,5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8, _
8,8,8,8,9,9,9,9,10,10,10,10,10,11,11,11,11,12,12,12, _
13,13,13,13,14,14,14,15,15,15,16,16,16,17,17,18,18,18, _
19,19,20,20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28, _
29,29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,42, _
43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,62,63,64, _
66,67,69,70,72,74,75,77,79,80,82,84,86,88,90,91,94,96,98, _
100,102,104,107,109,111,114,116,119,122,124,127,130,133,136, _
139,142,145,148,151,155,158,161,165,169,172,176,180,184,188, _
192,196,201,205,210,214,219,224,229,234,239,244,250,252,253

it won't compile (compiler says stack over flow)


So if there are only 255 values being stored in my eeprom, then why won't it let me squeeze in a byte to the final (256th byte) eeprom slot ...rather than defaulting the final value to 255?

Any ideas?

[RossWaddell]

Only if using PWM on a PIC pin , you can directly drive a transistor base without a resistor.
That will save you one component (resistor) and also space on PCB.

If you use Mosfet with PWM, you can also directly drive the gate without resistor. But this time, you need to have a resistor between the gate and GND. This resistor is not needed if not using PWM.

Hi sayzer - I've been working on driving up to 10 LEDs with a BS170 MOSFET and have connected the PIC pin directly to the gate of the 'FET. I had seen a circuit diagram with a resistor connecting the PIC -> gate to end, but then heard that this wasn't needed. What size resistor should I use?

[Ioannis]

Not necessary to add resistor to Fet gates.

Only if you had bipolar transistors.

Ioannis

[RossWaddell]

Thanks heaps, Ioannis.