Hi, i use the ADS1299 in the attached layout.I use an external Oscillator that can be enabled with the ADS1299's GPIO2. CLK_SEL is driven by GPIO3. For now i don't want to use the external oscillator, so i don't touch the GPIO registers or the CLK_EN bit. As the datasheet says, the GPIO's are configured as Inputs after startup. But in my case GPIO3 pulls down CLK_SEL if they are connected on startup. Since CLK_SEL is LOW now but the oscillator isn't enabled, the chip doesnt work (or talk via SPI). If i cut that wire and start the board, then connect GPIO3 to CLK_SEL while it is running, it stays HIGH (by the pull-up resistor). Thanks for any help. Kind regards Rick
https://www.dropbox.com/s/k4r8mfmifmnwkjd/schematic.png