
Arch
8 Wed May 28 17:36:23 1997
Draft 1/21/97
2-8
Architecture
PRE.4 for Rev. D
Copyright 1997 by LSI Logic Corporation. All rights reserved.
At the same time the PCR is extracted from the transport packet, the
value of the on-chip 42-bit LMC counter is extracted to a 42-bit register,
LMCR. The LMCR is the combination of the LCM’s Low, Middle, and
High registers. The LMC is a counter that runs on the local master clock
input driven by the external VCxO device.
The PCR and LMCR are registers the host processor reads when it
closes the control loop of the frequency lock, or writes to start and stop
the LMC count. When the PCR is extracted, an interrupt can be gener-
ated to the host processor. The host processor reads the PCR and
LMCR values, which are input parameters to the host programmable dig-
ital loop lter.
The LMCR registers can be written to by the host CPU to control starting
and stopping of counting. When the host CPU writes to LMC_HIGH
(0xfffe5), or LMC_MID (0xffe6), the LMC stops counting. To resume
counting, the host CPU writes to the LMC_HIGH (0xffe7) register.
The host CPU reads a free-running LMC by setting a new bit 15 in the
PCR_PID register with LMC_SEL=1. Once read, the LMC register bit is
reset to zero, to alleviate an extra reset of the PCR_PID register. If the
LMC_SEL=0, the host CPU reads the previously latched LMC value.
The digital loop lter calculates the error between the values read from
PCR and those from LMCR. The magnitude and polarity of the error
determine the correction value that is programmed into the on-chip 16-
bit Sigma Delta Register (SDR). The differences between every two con-
secutive PCRs and LMCRs are calculated to compensate for starting
point offset between the PCR and the LMCR. The following formula is
used to calculate the error in ppm.
Equation 2.1
Figure 2.6 shows how the frequency error value enters a digital loop lter
program, which outputs ltered values for the Sigma Delta circuit. A sim-
ple digital lter that can be implemented is an averaging lter. More com-
plicated lters can be implemented when operating in environments with
signicant jitter.
PCR
LMCR
---------------------- 1
–
106
×
FErr
=