LOCK AND SETTLING TIME CONSIDERATIONS FOR Si5324/27/ 69/74 ANY-FREQUENCY JITTER ATTENUATING CLOCK ICS 1. Introduction As outlined in the Product Bulletin*, issued in January 2013, Silicon Labs has made a change to the Si5324/27/69/ 74 family to improve the frequency locking time. However, in some instances, the time required to clear the Lossof-Lock (LOL) alarm may be longer in some operating configurations on the new devices. The purpose of this application note is to define the differences between LOL Clear Time and frequency locking time, how they may have been affected by the change, and how to optimize the LOL time in relation to the frequency settling time. 2. Lock Indication vs. Frequency Settling Time A phase lock loop (PLL) is a feedback system that strives to set the output frequency equal to that of the input frequency, as determined by the divide-by ratio. There are several states that a PLL can be in: unlocked, an in-between state (such as acquiring lock state), and finally a locked state. Also various terms can be used to describe the time for a PLL to acquire a locked status, such as acquisition time, lock time and settling time. There are also several ways to measure lock, phase vs. frequency, plus the degree of resolution it is measured to, eg., to within 5 ppm or 5 ppb of the final value. All of these variations impact the final result. There are conditions in which a PLL is unquestionably locked. There are also conditions which a PLL is unquestionably unlocked. However, there are several other states, such as acquiring lock, tracking an input change, locked but wandering, locked but unstable, etc. Whereas a lock condition can have various degrees of being locked, such as an analog function, the Loss of Lock output is digital: the PLL is locked or the PLL is unlocked. The Si53xx family LOL algorithm works by continuously monitoring the phase of the input clock in relation to the phase of the feedback clock. A retriggerable one-shot is set each time a potential phase cycle slip condition is detected. If no potential phase cycle slip occurs for the retrigger time, the LOL output is set low, indicating the PLL is in lock. 3. Product Update s Effect on PLL Parametrics During the acquisition phase a lock detect circuit can oscillate between reporting a locked and unlocked condition as the PLL transitions from slightly high to slightly low in frequency, as shown in Figure 1. Silicon Labs has recently introduced a modification to stabilize LOL by forcing it to a logic high state, which will result in a decrease in frequency settling time. This improvement is required in low loop BW applications only, which include the Si5324, Si5327, Si5369, and Si5374. Figure 2 shows the improved LOL condition due to the modification. Note: * Product Bulletin 1301281: Si5324/27/69/74 Improved Lock Time, issued on 28-Jan-2013, is available and automatically distributed to registered users of www.silabs.com. Rev. 0.1 12/13 Copyright 2013 by Silicon Laboratories AN803
Figure 1. Extended LOL, Low Loop BW Figure 2. Improved LOL Time Lock time would normally be much longer for a low loop BW setting, such as 4 Hz, and on the order of several seconds to minutes. However, this length is intolerable in some applications, which require a minimal lock time. The Si53xx family includes a fast lock feature which helps provide a fast lock time for low loop BW applications. The Fastlock feature sets the PLL loop BW to a maximum value during the initial acquisition cycle, or an ICAL command, and reverts back to the low loop BW once the LOL is released. The higher loop BW, as set by the fast lock feature, will also be active once the Si5324/69/74 has entered Digital Hold (provided that Digital Hold Valid is valid) to aid relocking times, and once again it reverts back to a low loop BW when the PLL relocks. As a result, lock times actually decreased, as seen when comparing Figure 1 and Figure 2. However, in some applications, there will be an increase in LOL Clear Time which is unwarranted (see Figure 3 and Figure 4). There is an easy modification that can be used to revert back to pre-modification performance. 4. Impact of System Level Changes At the system level, LOL can be used to determine a locked condition before a system is allowed to pass traffic. For some frequency plans, the LOL time has decreased, as shown in Figure 1 and Figure 2, which should not cause any issues. For the frequency plans with an increased LOL time, passing diagnostic testing will require a longer period of time. An example of the LOL measurement is shown in Figure 3 and Figure 4. Figure 3 shows the LOL time for an Si5324 in a 19.440 MHz to 155.520 MHz application with the lowest loop BW setting before the modification. Figure 4 shows the LOL time after the modification. We see an increase in LOL time from 3.8 seconds to 13 seconds. However, when the frequency settling time is measured, as in Figure 5, we see the performance has improved from 2.5 seconds to under 1 second.the overall or real lock time performance has improved. In this example, it is safe to make adjustments that revert LOL times back to the pre-modification performance. 2 Rev. 0.1
Figure 3. LOL Time Before Modification Figure 4. LOL Time After Modification Rev. 0.1 3
25 Frequency Settling Time m p 20 15 10 5 0 5 10 15 20 25 0 1 2 3 4 5 Time in Seconds Pre Modification Series1 Post Series2 Modification Figure 5. Modification Frequency Improvement 4 Rev. 0.1
5. Optimizing LOL Clear Time Using the LOCKT Register Setting The LOL active high state time can be modified by making adjustments to the LOCKT function. Decreasing the LOCKT time will also decrease the LOL active high time, however it does not change the frequency settling time. In the above example, LOCKT was modified from 53 ms to 13 ms and LOCKT was restored back to 3.7 seconds as shown in Figure 7. If LOCKT is set too low in value then it can cause an early release of fast lock resulting in extended lock times. Table 1 show the LOL time versus LOCKT. In this example, the frequency settling time was about 1 second and modifying the LOCKT to 6.6 ms would be a conservative setting. Note these figures are provided for informational purposes only and actual results will differ depending on the frequency plan. Table 1. LOL Time vs. LOCKT Setting, 19.440 to 155.520 MHz LOCKT Register Setting Register 19[2:0] LOCKT Setting LOL Time (in seconds) Frequency Settling Time (in seconds) 0x0 100 ms 25.2 1 0x1 53.3 ms 12.9 1 0x2 26.5 ms 6.3 1 0x3 13.3 ms 3.7 1 0x4 6.6 ms 2.2 1 0x5 3.3 ms 1.4 1 0x6 1.66 ms 1.0 1 0x7 0.83 ms 0.83 1 Rev. 0.1 5
6. Modifying LOCKT LOCKT can easily be modified by using DSLLsim. After starting up the latest revision of DSPLLsim, select Enter File, select Read from register map test file and enter the current frequency plan, select the Input Clocks tab and modify LOCKT as shown in Figure 6. Resave the register file. LOCKT can also be modified by changing the values in register 19 [2:0]. See the appropriate data sheet for more details. Figure 6. LOCKT Modification Location, Select Input Clocks Tab in DSPLLsim 6 Rev. 0.1
Figure 7. LOL Time is Restored Back to Less than 4 Seconds with a LOCKT Modification Rev. 0.1 7
7. Verifying the NVM Revision In some cases, it will be helpful to query the Si53xx to determine which NVM version is loaded. The purpose of this query is to identify the device s vintage and from this the appropriate LOCKT setting can be selected for use in the register plan. Register 185 contains which NVM revision is used and this applies for all Si5324s, Si5374s, Si5368s and Si5374s. Table 2 contains the NVM revision pre- and post-product Bulletin 1301281. Table 2. NVM Revision Pre- and Post-Modification, Register 185 Device NVM before 5/1/2013 as well as three new part numbers 1 Si5324E-C-GM Si5374C-A-BL Si5374C-A-GL NVM on or after 5/1/2013 Si5324, Register 185 13h 16h Si5327, Register 185 13h 16h Si5369, Register 185 13h 16h Si5374, Register 185 14h 16h Notes: 1. These three specific new part numbers were created for applications which require pre-modification performance. Contact applications engineering with a detailed frequency plan if there are any concerns with the LOL improvement and for recommendations as to LOCKT modifications. 8 Rev. 0.1
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