Electronic – How to drive LED when sinking current to MCU

gpioledpic

Hopefully a simple one. Should I always control by writing to the GPIO digital output? Or should I always leave it low and then tri-state the output?

LED turns on when MCU pin sinks current:

enter image description here

In code, I mean the difference between the following for the given schematic:

// Control entirely via GPIO    
TRISBbits.TRISB5 = 0; // configure as digital output
LATBbits.LATB5 = 0; // turn on the LED
LATBbits.LATB5 = 1; // turn off the LED

// Tri-state output to always just enable/disable
LATBbits.LATB5 = 0; // drive output low
TRISBbits.TRISB5 = 0; // turn on by configuring as digital output
TRISBbits.TRISB5 = 1; // turn off by configuring as digital input

Best Answer

In the case you've shown, it doesn't really matter too much. I'd leave the output as an output and control the LED from the LAT bit. If the input is not a Schmitt Trigger input, you could get unnecessary power consumption from having an input floating around, and having inputs floating around is just not good practice in any case.

Where it could matter more is if you're using an older PIC that does not have a separate LAT register (or are directly writing to the port register on a PIC that has a LAT register, in which case, don't do that).

In such a case, you could run into a read-modify-write issue where an instruction to modify a different (unrelated) bit in the same port register could flip the LED port bit into a high state, so the next time you set the TRIS bit low the LED does not turn on. It might not happen reliably one way or the other since the pin is floating around.

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Please look at page 8 of the datasheet under "Dimming Control". You can do either PWM or analog dimming. For analog, you vary the EN/DIM pin between 0.7V (0%) and 1.4V (100%). For PWM, you use a 1HKz signal with on > 1.4V and the duty cycle controls the brightness.

You can do it either way.

And to answer your question, putting 5V into EN should give you 1A to the LEDs continuously.

Electronic – Control multiple LEDS with one PWM pin

Charlieplexing my friend.

You simply hook them up like this:

Then you write high and low on the different pins which will activate individual LED's.

Let's say you want LED3 to light up, then you set it up like this:

Pin1 = INPUT //high impedance
Pin2 = OUTPUT(1)
Pin3 = OUTPUT(0)

If you want Led3 to be 50% bright, then implement a PWM in software.

Pin1 = INPUT //high impedance
Pin2 = software_pwm(0.5)
Pin3 = OUTPUT(0)

And if you want all 4 to light up (I assume that you don't connect LED5 and LED6)

Then it will look like this:

while(1){

    //LED1
    Pin1 = OUTPUT(1)
    Pin2 = OUTPUT(0)
    Pin3 = INPUT  //high impedance
    delay_some_milliseconds()  
    //we don't need to be flashing at 500khz, or whatever high
    //frequency we'll get without any delays, 60Hz is good enough,
    //some ms will bring you down there. 

    //LED2
    Pin1 = OUTPUT(0)
    Pin2 = OUTPUT(1)
    Pin3 = INPUT
    delay_some_milliseconds()

    //LED3
    Pin1 = INPUT
    Pin2 = OUTPUT(1)
    Pin3 = OUTPUT(0)
    delay_some_milliseconds()

    //LED4
    Pin1 = INPUT
    Pin2 = OUTPUT(0)
    Pin3 = OUTPUT(1)
    delay_some_milliseconds()
}

LED turns on when MCU pin sinks current:

Best Answer

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