How to access clk_sel on the 4ch RP in C to select an external CLK

[arparker]

I'm just starting to play either the 4 channel RP and would like to use an external clock to clock the ADC. On the previous board I would simply move the resistors on the circuit board but for the 4CH version I notice on the schematic that a provision for a software selection was implemented to allow the developer to select which clock. Can anyone point me in the right direction of where this is mapped in the registers?

[juretrn]

Hi,
the CLK_SEL port is tied to the expansion connector E2 - pin 21.
The idea behind this is that Red Pitaya will soon offer a clock distributor shield that will enable low jitter clock inputs, and the external clock will come through that.
That means this pin must be changed in hardware (I don't know if it must go to high or low level, though) if you want to use an external clock.

[arparker]

Thanks for the response. I think this will be a good addition to the RP capabilities. On the schematic I don't see a part number associated with the LVDS switch that's connected to the CLK_SEL line. Do you have a part number so that I can research this part and determine how to drive the line in hardware?

[redpitaya]

Hello Arparker,

We can save you a bit of time on this. Connect the CLK_SEL pin to GND for the 4 Input version to switch to EXT clock and to 3V3 (Vcc) to use internal clock.

[arparker]

Great, thanks for the quick response. This makes life easier for sure.

[fbalakirev]

Red Pitaya will soon offer a clock distributor shield that will enable low jitter clock inputs, and the external clock will come through that.

Where can I find more info about a clock distributor shield with low jitter clock inputs?

[juretrn]

It was just released

https://go.redpitaya.com/red-pitaya-click-shield

[fbalakirev]

It was just released

https://go.redpitaya.com/red-pitaya-click-shield

Thank you for the link! We ordered a few to try them out.

[fbalakirev]

We received the click boards and the boards when installed completely cover the CPU fan. We currently use 30x30mm 10mm tall 5V Dc fans powered by Red Pitaya. We also tried Red Pitaya aluminum enclosure, but we find that CPU runs about 15-20 degrees hotter with enclosure compared to fan.

Do you happen to have alternative cooling solutions?

[redpitaya]

We have the Heatsink interface - https://redpitaya.com/product/heatsink-interface/
You can attach it to a bigger metal enclosure to keep the whole board cool. You can also cool it with a fan. The aluminium case can also be attached to an external cooling heatsink to further reduce the heat, but it might be a bit more tricky as it does not have predrilled holes.