Have you ever been frustrated just trying to get your phase-locked loop (PLL) to lock? Premature assumptions can make the debugging process much longer and more tedious than necessary. By following the processes outlined here for verifying communication and establishing lock, the debugging process can be simplified.
Step 1: Verify communication
The first step is to verify the PLL’s ability to respond to programming. If the PLL is not locking and you cannot read back from it, try sending software commands that require a minimum amount of hardware commands to work. One possibility is to power the PLL up and down via software (not pin) and look for a predictable current change or bias-voltage-level change on a pin. Many PLLs have a bias level on their input (OSCin) pin that is Vcc/2 when powered up and 0V when powered down.
If the PLL has integrated voltage controlled oscillators (VCOs), look at the low-dropout (LDO) output pin voltages and see if they react to power-down and power-up commands. It might be possible to toggle input/output (I/O) pins such as the MUXout pin on many LMX PLLs. If you can verify communications with these methods, then go ahead and try to achieve lock.
If you cannot verify communication, then look for common culprits such as the following:
Step 2: Establish lock
Once you have verified communication, the next step is to try to achieve lock with the PLL. These are some of the more common reasons for an unlocked PLL:
The debugging process for getting a PLL to lock can be much simpler by following a systematic approach and not making premature assumptions. Figure 1 shows a flowchart that can be used to guide one through this process.
Figure 1: PLL debugging flowchart
If you would like to learn more about the challenges of unlocked PLLs, check out the Texas Instruments portfolio of phase lock loops.
This is very good initial trouble shooting way!!
Can I download and print out your book PLL performance, simulation and design BOOK ?
Thank you for your interest in my PLL book. I am finishing up my 5th edition in the next few weeks, but a draft version is available for download at:
In about 3 weeks, I should have the final version, but it will not be available to print out; if you want a printed copy, this will be available through book retailers, like Amazon.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.