Device Architecture
PLL Macro
The PLL functionality of the clock conditioning block is supported by the PLL macro. Note that the
PLL macro reference clock uses the CLKA input of the CCC block, which is only accessible from the
global A[0:2] package pins. Refer to Figure 2-22 on page 2-28 for more information.
The PLL macro provides five derived clocks (three independent) from a single reference clock. The
PLL feedback loop can be driven either internally or externally. The PLL macro also provides power-
down input and lock output signals. During power-up, POWERDOWN should be asserted LOW until
VCC is up. See Figure 2-19 on page 2-25 for more information.
Inputs:
•
•
CLKA: selected clock input
POWERDOWN (active low): disables PLLs. The default state is power-down on (active low).
Outputs:
•
•
•
LOCK (active high): indicates that PLL output has locked on the input reference signal
GLA, GLB, GLC: outputs to respective global networks
YB, YC: allows output from the CCC to be routed back to the FPGA core
As previously described, the PLL allows up to five flexible and independently configurable clock
outputs. Figure 2-23 on page 2-29 illustrates the various clock output options and delay elements.
As illustrated, the PLL supports three distinct output frequencies from a given input clock. Two of
these (GLB and GLC) can be routed to the B and C global networks, respectively, and/or routed to
the device core (YB and YC).
There are five delay elements to support phase control on all five outputs (GLA, GLB, GLC, YB, and
YC).
There is also a delay element in the feedback loop that can be used to advance the clock relative to
the reference clock.
The PLL macro reference clock can be driven by an INBUF macro to create a composite macro,
where the I/O macro drives the global buffer (with programmable delay) using a hardwired
connection. In this case, the I/O must be placed in one of the dedicated global I/O locations.
The PLL macro reference clock can be driven directly from the FPGA core.
The PLL macro reference clock can also be driven from an I/O routed through the FPGA regular
routing fabric. In this case, users must instantiate a special macro, PLLINT, to differentiate it from
the hardwired I/O connection described earlier.
The visual PLL configuration in SmartGen, available with the Libero IDE and Designer tools, will
derive the necessary internal divider ratios based on the input frequency and desired output
frequencies selected by the user. SmartGen allows the user to select the various delays and phase
shift values necessary to adjust the phases between the reference clock (CLKA) and the derived
clocks (GLA, GLB, GLC, YB, and YC). SmartGen also allows the user to select where the input clock is
coming from. SmartGen automatically instantiates the special macro, PLLINT, when needed.
2-30
Preliminary v1.7
ACTEL [ Actel Corporation ]
