Power, Reset, and Clock Management
All clock outputs of the DPLL can be gated. The Control module provides the DPLL with a clock gating
control signal to enable or disable the clock, and the DPLL provides the PRCM module with a clock
activity status signal to let the PRCM module hardware know when the clock is effectively running or
effectively gated. Output clock gating control for various clockouts:
CLKOUTEN/CLKOUTLDOEN/CLKDCOLDOEN.
8.1.6.4.1 Clock Functions
Table 8-19. Output Clocks in Locked Condition
Pin Name
Frequency
CLKOUT
[M /(N+1)] * CLKINP * [1/M2]
CLKOUTLDO
[M /(N+1)] * CLKINP * [1/M2]
CLKDCOOUT
[M /(N+1)] * CLKINP
Table 8-20. Output Clocks Before Lock and During Relock Modes
Pin Name
Frequency
Comments
CLKINP/(N2+1)
ULOWCLKEN=’0’
CLKOUT
CLKINPLOW
ULOWCLKEN=’1’
CLKDCOLDO
LOW
CLKOUTLDO
LOW
8.1.6.5
M2 and N2 Change On-the-Fly
The dividers M2 and N2 are designed to change on the fly and provide a glitch-free frequency switch from
the old to new frequencies. In other words, they can be changed while the PLL is in a locked condition,
without having to switch to bypass mode. A status toggle bit will give an indication if the new divisor was
accepted. These dividers can also be changed in bypass mode, and the new divisor value will be reflected
on output after the PLL relocks. For more details, see the PLL configuration procedures for each PLL.
8.1.6.6
Spread Spectrum Clocking (SSC)
The module supports spread spectrum clocking (SSC) on its output clocks. SSC is used to spread the
spectral peaking of the clock to reduce any electromagnetic interference (EMI) that may be caused due to
the clock’s fundamental or any of its harmonics. When SSC is enabled the clock’s spectrum is spread by
the amount of frequency spread, and the attenuation is given by the ratio of the frequency spread (
Δ
f) and
the modulation frequency (f
m
), i.e., [{10*log10(Df/f
m
)}-10] dB.
SSC is performed by changing the feedback divider (M) in a triangular pattern. Implying, the frequency of
the output clock would vary in a triangular pattern. The frequency of this pattern would be modulation
frequency (f
m
). The peak (
Δ
M) or the amplitude of the triangular pattern as a percent of M would be equal
to the percent of the output frequency spread (
Δ
f); that is,
Δ
M/M=
Δ
f / f
c
. Next mark with Finp the
frequency of the clock signal at the input of the DPLL. Because it is divided to N+1 before entering the
phase detector, so the internal reference frequency is Fref = Finp / (N + 1).
Assume the central frequency f
c
to be equal to the DPLL output frequency Fout, or f
c
= Fout = (Finp / (N +
1)) * (M / M2). Since this is in band modulation for the DPLL, the modulation frequency is required to be
within the DPLL's loop bandwidth (lowest BW of Fref / 70). A higher modulation frequency would result in
lesser spreading in the output clock.
SSC can be enabled/disabled using bit CM_CLKMODE_DPLL_xxx.DPLL_SSC_EN (where xxx can be
any one of the following DPLLs: MPU, DDR, DISP, CORE, PER). An acknowledge signal
CM_CLKMODE_DPLL_xxx.DPLL SSC_ACK notifies the exact start and end of SSC. When SSC_EN is
de-asserted, SSC is disabled only after completion of one full cycle of the triangular pattern given by the
modulation frequency. This is done in order to maintain the average frequency.
523
SPRUH73H – October 2011 – Revised April 2013
Power, Reset, and Clock Management (PRCM)
Copyright © 2011–2013, Texas Instruments Incorporated