C O U N T
C A P T U R E
S S W
C O U N T
C L K O U T
S S W
O D
N R
N F
_
_
_
_
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ø
ö
çç
è
æ
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´
-
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COUNT
CLKOUT
SSW
COUNT
CAPTURE
SSW
NR
abs
Depth
_
_
_
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2
1
PLL
533
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
14.5.5 PLL Modulation Depth Measurement
The PLL contains a circuit for estimating the depth of the modulation. The circuit counts clock edges over
a fixed window of the modulation waveform (SSW_CAPTURE_COUNT in SSWPLL2) and clock edges
over the entire waveform (SSW_CLKOUT_COUNT in SSWPLL3). The capture ends after a pre-
determined number of clock edges in SSW_CLKOUT_COUNTER as set in TAP_COUNTER_DIS. There
are 2 × NR windows per modulation waveform. The procedure for estimating the modulation depth is:
1. While GCLK1 is sourced by the oscillator and the PLL is enabled with modulation, configure SSWPLL1
as follows:
a. CAPTURE_WINDOW_INDEX is set equal to NR.
b. COUNTER_RESET is set.
c. TAP_COUNTER_DIS is set to disable the measurement after SSW_CLKOUT_COUNT captures
this number of clocks. The measurement is disabled after the set tap is set AND the modulation
cycle ends.
d. Ensure that EXT_COUNTER_EN is cleared.
2. Ensure that both SSW_CAPTURE_COUNT and SSW_CLKOUT_COUNT are cleared (by the
COUNTER_RESET).
3. Set COUNTER_EN and clear COUNTER_RESET. This step releases the reset and enables the
counter to begin counting.
4. After a wait loop, poll for COUNTER_READ_READY to set. After the bit is set, read
SSW_CAPTURE_COUNT and SSW_CLKOUT_COUNT.
5. Compute the modulation depth as:
(12)
14.5.6 PLL Frequency Measurement Circuit
The same circuit that is used to measure modulation depth is also available to measure the average
frequency of the PLL. In this mode, the PLL output (before the R-divider) is captured in
SSW_CLKOUT_COUNT while the oscillator is captured in SSW_CAPTURE_COUNT. The procedure for
using the PLL frequency measurement circuit is:
1. While the PLL is enabled, set EXT_COUNTER_EN.
2. Set COUNTER_EN. This bit clears both SSW_CAPTURE_COUNT and SSW_CLKOUT_COUNT and
then immediately enables for counting.
3. Wait for some software delay loop.
4. Clear COUNTER_EN. Wait for COUNTER_READ_READY to set. Read both
SSW_CAPTURE_COUNT and SSW_CLKOUT_COUNT and compute the ratio of PLL multiplication
as:
(13)
5. Note that CAPTURE_WINDOW_INDEX, COUNTER_RESET, TAP_COUNTER_DIS are not used in
this procedure
14.5.7 PLL2
PLL2 drives GCM clock source 6.
The PLL is identical to PLL1, except modulation is disabled on this instance of the PLL. Also, the PLL
typically does not clock the system, there is no automatic switch over feature. Any PLL error can be
handled by the CPU.
PLL2 is programmed through PLLCTL3.