Chapter 10 Memory Subsystem.pdf

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10.2.4.4.11 Controlled Delay Line
The DLL/CDL module is a hard macro composed of one master delay-locked loop (DLL) and five
slave-controlled delay lines (CDL). It is used to generate precise delays suitable for DDR read and write
operations. DLL delays are tracked at high frequency on process dispersion, and voltage and temperature
variations (PVT) .
A CDL is a component with a clock input signal, a clock output signal, and a delay value input. The output
signal is the input signal delayed according to the delay value.
The DLL output is a command that controls the CDLs (plus the controlled voltage) and assures an output
signal with 90-degree delay with respect to its input signal. The DLL contains five CDL blocks.
10.2.4.4.11.1 Purpose of the DLL/CDL Module
In DDR applications, the DLL and CDL combination helps provide a data strobe (DQS) with a delay
suitable to the main read and write RAM operations that exceed 83 MHz. The DLL functions within a
locking range of 83 to 200 Mhz. Below 83 MHz, the DLL must be used in unlocked mode. For lower clock
frequencies, set the DLL to bypass mode.
DLL/CDL is used to delay the incoming DQS in case of DDR read, or delay the output DQ (data lines) in
case of DDR write (and, hence, to increase the ac timing margin). DQS is used only with DDR memory.
There is no need to use the DLL/CDL for the SDR DRAM because data is strobed every clock cycle. The
DQS signals are left unconnected for SDR SDRAM memories.
DQS is propagated with the data (thus reducing the impact of the propagation delay) and is used by the
receiver to sample the data.
The DLL/CDL combination minimizes the negative effects caused by skews and jitters of clock signals.
The delay introduced by the CDL base unit depends on PVT conditions. Moreover, the CDL timing delay
is not a linear function of the DLL counter offset. By means of the DLL feedback loop, the delay value is
updated in real time and is adjusted according to voltage and temperature variations.
DDR interfaces transmit data on both edges of the DQS bidirectional data strobe. Address and control
signals transmit at half the data frequency (that is, at the DDR clock frequency) and latch only on the
rising edge of the transmit clock.
The bidirectional data strobe (DQS) is transmitted externally, along with data, for use in data capture at
the receiver. sdrc_dqs[3:0] is an SDRC I/O that connects the SDRC with DDR SDRAM DQS pins. See
Figure 10-43 for an overview of DDR SDRAM connection with the SDRC. DQS is transmitted by the DDR
SDRAM during reads and by the SDRC during writes. DQS is edge-aligned with data for reads and
center-aligned with data for writes, as shown in Figure 10-57.
Figure 10-57 shows the generic DDR data-write and data-read waveforms.
2242 Memory Subsystem SPRUGN4L–May 2010–Revised June 2011
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