I have a self made LMX2492 PLL circuit that I cannot get to lock.
1. The SPI communication seems to work OK.
I send SWRST : 0x000204
I set POWERDOWN to "Power up, ignore CE" : 0x000201
I config MUXout_MUX to "output readback" and set to MUXout_pin to "Pullup/Pulldown output" : 0x00273a
At this time I am able to read back the value of any register, ie. if I send 0x801000 (the PLL_N[0:7] register) I can see 0x64 on the MUX_out pin using an oscilloscope.
Just to be complete, after each of these SPI messages the LE line is pulsed.
So far so good.
2. Next I try to read back the R/2 counter to check if the reference oscillator input have propagated to the PFD.
The SPI programming is as follows
SWRST : 0x000204
POWERDOWN : 0x000201
MUXout_MUX set to R/2 and MUXout_pin set to Puup/Pudn : 0x002782
It is my understanding of the datasheet that I at this stage should see 25 MHz signal at the MUX_out pin (my input oscillator has frequency 50 MHz). Sadly it is dead 0 Volt.
I measure the differential voltage between OSCin and OSCin* (pin 9 and 10) to 0.9 Vpp. According to the datasheet section 7.6 VOSCin must at least be 0.5 Vpp.
Hence, my input signal should have sufficient magnitude.
I have also tried to read back N/2 (ie. program 0x002792) with the same result. In this case it is more difficult to accurately measure if the input signal has sufficient power (> -5 dBm)
although I think this is the case.
3. My next effort was to read back the TRIG1 input.
Again, it is my understanding of the datasheet that if I program MUXout_MUX to "Input TRIG1", the logic value on the TRIG1 input should be relayed to the MUX_out pin.
My third SPI message in this case reads : 0x00270a
After this I toggle the TRIG1 input a number of times as I measure the MUX out pin. Once again, absolutely no action.
What could be the problem here ? All power and GND pins seems to have correct voltages.
1. Errors in my SPI programming
2. Soldering problems. I measure the voltages on the nets leading into the chip. It is difficult to measure directly on the pin on a chip scale package.
3. The IC is partly malfunctioning.
Item 2 and 3 requires de-soldering. Before I try that I wanted to ask if there are any other possibilities. Are there any more things I could try first ?
Thanks.