Why is the builtin LED blinking when playing from SD?

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Jeroen

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I have noticed that the little orange LED on my T4.0 blinks while audio samples from the SD card are playing.

It is actually kind of cool as it's in sync with the audio samples (or so it appears). I'm guessing this is because the LED is on pin 13 and sharing with SD SCK. looks the the led blinks in sync with SD reads or something.

Questions:

What's causing this?

How do I turn off the blinking?

Is there a way to tap into this and output the same signal to an external LED?

Tia,

Jeroen
 
Yes, The blink on Pin 13 - the LED pin is because SD uses SPI bus and the Pin 13 is the SCK or Clock pin for the SPI transfers.

The T_4.0 doesn't have an alternate pin for SCK.

Might be possible to duplicate that LED blink to another LED - but it runs fast 25 MHz or so - and if the electrical characteristics change much it won't present the needed clean clock edge for proper function.

It does have bottom pads to bring out SDIO for 4 bit SD access - but that isn't a trivial adventure to replicate what the T_4.1 has included on the Teensy.
 
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