8 replies to this topic

[Anshul]

I have been scouring the deepest reaches of the internet for about a week trying to find a working example for using the TLC5940 16-channel PWM driver chip for dimming LED strips, but without any luck. Most of the examples are either incomplete (missing the wiring diagram) or just don't work. Here's a few examples that I couldn't get to work: 

 

1. http://forums.netdui...c/4123-tlc5940/

2. http://forums.netdui...940-pwm-driver/

 

All of these are old, so I'm fairly certain they target .NETMF 4.1. The first one didn't work (http://forums.netdui...c/4123-tlc5940/) because it uses PWM.SetPulse from .NETMF 4.1 which I tried to convert the code to 4.2, but it's still not working.

 

Here is my request: Can someone help me out with a program that simply fades an LED from 0 to full brightness and back to 0, and then repeats, using the TLC5940? 

 

Once I can get a single LED to PWM using the TLC5940, the rest of the work is already done and I have the high-power drivers for the LED strips already built. Thanks for the help.

[NooM]

well, the tlc5940 is a bit a pain with netmf. it needs timing, wich would be nice in native with a timer (thats what i do on arduino)

- its just a mix of a shift register and a counter

 

theres different tlc59xx versions wich dont need a gs clock and a blank signal. they work great for led stuff, but not for led matrixes (flickering cos you cant control the blank line - timing issues)

 

 

btw: in the second links theres a link to nevyn's blog, with some code. that should work "ok" (i still see a timing issue with an external counter like that - you normally sent the new data directly after you blanked the chip)

[Anshul]

Do you know which chips do not require an external clock signal? The blog did have a wiring diagram but I don't have the components for creating a clock. Can you post any details on how you implemented it? Especially the wiring diagram and the basic algorithm. Thanks.

[Nevyn]

Do you know which chips do not require an external clock signal? The blog did have a wiring diagram but I don't have the components for creating a clock. Can you post any details on how you implemented it? Especially the wiring diagram and the basic algorithm. Thanks.

 

For LEDs you can get away without the clock as an external circuit by using PWM.  You really need two signals for the clock and blank control.  The grey scale clock controls the counter inside the chip and the blank signal resets the counters and starts the count sequence again.

 

Try setting up a PWM signal of say 1 MHz (or some other suitably high value) with a 50% duty cycle.  This will be the greyscale clock.

 

Once this is working setup a second PWM which is a single pulse.  These pulses will occur at the greyscale clock frequency from above divided by 4096.  The pulse should be very narrow.

 

Although this will not give an ideal clock for the chip it will be accurate enough for LED control.

 

Regards,Mark

[Anshul]

Once this is working setup a second PWM which is a single pulse.  These pulses will occur at the greyscale clock frequency from above divided by 4096.  The pulse should be very narrow.

 

How do you do this on the Netduino in NETMF 4.2? You can change the frequency and/or period but how would you send a single pulse? 

[Anshul]

 

Once this is working setup a second PWM which is a single pulse.  These pulses will occur at the greyscale clock frequency from above divided by 4096.  The pulse should be very narrow.

 

 

 

Is the second PWM the latch signal?

 

I found a programming flowchart (http://www.ti.com/li...106/slvc106.pdf) that I've been following, but I'm still confused as to how exactly the values are moved inside the TLC5940. If some values are shifted out to the outputs, does that output constantly output a PWM signal with that duty cycle?

 

For example if I write out 011111111111 (binary) = 2048 (dec) = 0.5 duty cycle to the first output (OUT0), after I clock out the values by PWMing GSCLK and then pulsing the latch once, will OUT0 output a 0.5 duty cycle until something changes? That's what I thought the chip does but maybe I'm wrong.

[Nevyn]

Is the second PWM the latch signal?

 

I found a programming flowchart (http://www.ti.com/li...106/slvc106.pdf) that I've been following, but I'm still confused as to how exactly the values are moved inside the TLC5940. If some values are shifted out to the outputs, does that output constantly output a PWM signal with that duty cycle?

 

For example if I write out 011111111111 (binary) = 2048 (dec) = 0.5 duty cycle to the first output (OUT0), after I clock out the values by PWMing GSCLK and then pulsing the latch once, will OUT0 output a 0.5 duty cycle until something changes? That's what I thought the chip does but maybe I'm wrong.

 

The second PWM signal is the "counter reset" signal.  If you do not pulse this pin then the counters start at 4095, count down to zero and then stop.  Pulsing the Blank pin tells the chip to restart.  That it why the pulses should come along every 4096 grey scale clock pulses.

 

The data is latched on the XLAT pulse.

 

Regards,

Mark

[Nevyn]

Is the second PWM the latch signal?

 

I found a programming flowchart (http://www.ti.com/li...106/slvc106.pdf) that I've been following, but I'm still confused as to how exactly the values are moved inside the TLC5940. If some values are shifted out to the outputs, does that output constantly output a PWM signal with that duty cycle?

 

For example if I write out 011111111111 (binary) = 2048 (dec) = 0.5 duty cycle to the first output (OUT0), after I clock out the values by PWMing GSCLK and then pulsing the latch once, will OUT0 output a 0.5 duty cycle until something changes? That's what I thought the chip does but maybe I'm wrong.

 

The second PWM signal is the "counter reset" signal.  If you do not pulse this pin then the counters start at 4095, count down to zero and then stop.  Pulsing the Blank pin tells the chip to restart.  That it why the pulses should come along every 4096 grey scale clock pulses.

 

The data is latched on the XLAT pulse.

 

Regards,

Mark

[Anshul]

The second PWM signal is the "counter reset" signal.  If you do not pulse this pin then the counters start at 4095, count down to zero and then stop.  Pulsing the Blank pin tells the chip to restart.  That it why the pulses should come along every 4096 grey scale clock pulses.

 

The data is latched on the XLAT pulse.

 

Regards,

Mark

 

That clears it up a bit more. One question though, when you say that the data is latched on the XLAT pulse, is that latching the data from the serial input to the GS register or from the GS register to the outputs?