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COUNT
CLKOUT
SSW
COUNT
CAPTURE
SSW
NR
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Depth
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1
PLL
382
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
10.5.5 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 GCLK 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)