MiSTer FPGA Forum

Hi,

Appreciate some help - I've been trying to debug the SPDIF optical output.


I have volumectl = 1, and am seeing PWM on the I2S header pin 1 which goes to the LED via 200 ohm resistor.
LED is on and analog audio is OK too.

a. Yellow waveform is the I2S pin
b. Green waveform is anode of the LED

Am testing this by feeding into a SRC-2496 which can't detect any signal. Syncs with the toslink from the HDMI just fine.

Have tried a couple of diodes without luck. Currently testing with 4303F1.

Thanks for any advice

I've just has a look at your website and saw your picture of a built IO board, are you mounting the LED in the correct orientation?

The TOSLINK LED should be mounted as shown below in my image. Hope this helps.

Thats Nat.. yeah that was an early photo.. it's properly lined up with the mini-Toslink cable that's plugged into the 3.5mm.


The light output isn't as bright as the Toslink coming out of my TV.. maybe I need to give the LED more current ?

Based on the measurements.. the LRCLK is 3.26v, the forward voltage is 2.17v so therefore current through the 200R is 5mA.
The diode is rated at 10mA.

https://docs.rs-online.com/3eb2/0900766b815c2413.pdf

The TOSLink LED should be from memory 650nM wavelength which looking at your spec sheet is correct however the brightness should be 80mcd otherwise you will experience problems.

If you PM me your address I am happy to send to a bag of a hundred of the ones I use for free, I have a couple of thousand here.

I put a 100R in to increase LED current.. looks like nominally about 10mA now but SPDIF still doesn't work. So will try a few more LEDs and see if I get any better results. Big thanks to Nat for sending me some of the ones he has which work.

Different LED models have different brightness, so it seems the LED used on your board is not suitable for TOSLink purpose.