Probe capacitance makes that tricky - if I put my probe on DSPEED, I see that it starts at approx. 2.5V then gradually decreases until it hits 0V. DTAP decreases from 3MHz to 0 during this time.
I'll try to get something together to show you.
Can you post a scope trace of your refclk together with DTAP, DSPEED and DENABLE?
Thanks for responding so quickly. Yes I have my clock source going to REFCLK+ (CLKIN is the signal name on my schematic). BIAS is 0.7V exactly, /RESET is high, A0-A3 are 0x0000, and the loop filter has a 4.7nF cap to GND with a 130ohm resistor + 1uF cap in parallel, just like the eval board.
Regarding the clock - I am not using an LVDS clock, but rather a 2.5V-level clock signal, with REFCLK- tied to 1.25V. Sheet 9 of the datasheet states: If no LVDS reference clock signal is available, a CMOS signal can be connected to REFCLK+ and the REFCLK input is connected to VDD/2 via a resistor divider.
Is that not a true statement?
I guess you mean "1 MHz clock at REFCLK+", and not CLKIN, there is no CLKIN, just a SRCLK, but that is someting else!
There could be many reasons why this is not working. It's hard for me to debug your board without actually having it in hands. So just some ideas:
- Supply a clean differential REFCLK, I never tried one end tied to VDD/2
- Is /RESET high?
- Is BIAS at roughly 0.7V?
- Is A0-A3 different from 1111, which puts the chip in standby
- Did you double check your loop filter?
The easiest usually is to start from a running evaluation board, then compare all pins 1:1 with your board.
I am working on a custom PCB design with the DRS4 chip, and I can't get the PLL to lock. I'm feeding CLKIN with a 1MHz CMOS clock (REFCLK- tied to VDD/2), and I'm using the same loop filter as the eval board. I see from the datasheet that the PLL is enabled by default, so I'm not writing anything to the config register on startup. I am just driving DENABLE high approx. 100ms after startup and looking for the PLL lock bit to go high. When I look at DTAP, I see a 3MHz signal. Can anyone tell me what I'm doing wrong?