urukul: add note on clk_div with pll disabled

src/hw/urukul.md

@@ -149,3 +149,24 @@ ValueError: Urukul AD9910 AUX_DAC mismatch
 ```
 
 Ensure it is the AD9910 and not the AD9912. Also check SUServo pins are set up respective to the JSON description.
+
+### Jagged signal with 1GHz external clock on AD9910
+
+By default, on AD9910 external clock signal is divided by 4, while it should be not divided at all with PLL disabled.
+Change the ``clk_div`` parameter to the CPLD in the device_db file:
+
+```python
+device_db["urukulX_cpld"] = {
+    "type": "local",
+    "module": "artiq.coredevice.urukul",
+    "class": "CPLD",
+    "arguments": {
+        "spi_device": "spi_urukul0",
+        "sync_device": None,
+        "io_update_device": "ttl_urukul0_io_update",
+        "refclk": 1000000000.0,
+        "clk_sel": 1,
+        "clk_div" : 1 # <--- add this line
+    }
+}
+```