help me understand

groebke1

I have a rev b eval board.
When I run the simple program below the terminal values are correctly adding 1 with each iteration.
When I look at the leds, the values are inverted from what I would expect. I would think writing a "1" to the out register should turn on the LED.

Can someone explain to me why the LED values are inverted?

Thanks in advance
Greg


_clkfreq = 10_000_000

pub go() | LEDs

repeat
LEDs += 1
debug(ubin(LEDs), "'")
pinwrite(56 addpins 7, LEDs)
waitms(2000)

Duane Degn

The LEDs on the evaluation board are active low.

There's a buffer between the P2 and the LEDs.

groebke1

Well thats an easy enough answer. Thanks quick answer.
Why would active low LEDs be desirable?

Duane Degn

groebke1 wrote: »

Why would active low LEDs be desirable?

I'm not sure but my guess would be they wanted an inexpensive buffer chip which wouldn't interfere with the I/O pins if the pins were used for other purposes.

FWIW, active low LEDs are pretty common.

Cluso99

Active low goes back in history when there was more drive current available by pulling a line low than driving it high.

And now, often LEDs have a voltage drop around 3V which means they need to be tied to 5V so that the resistor can be calculated to set the current reliably.

cgracey

I think in our case, we just spec'd an inverting 8-bit buffer. Maybe VonSarvas chose an inverting buffer because many of those top eight signals are active-low, like RX, TX, and SPI_CS. It's better for the lights to come on when something is happening, rather than during idle states.

Tubular

Those two leds on P56 / P57 on the Edge module, are those active low, or active high?

cgracey

Tubular wrote: »

Those two leds on P56 / P57 on the Edge module, are those active low, or active high?

They are active-high.