XPGA Datasheet, PDF (14/89 Pages) Lattice Semiconductor – The ispXPGA architecture

Lattice Semiconductor
ispXPGA Family Data Sheet
sysCLOCK PLL Description
The sysCLOCK PLL circuitry consists of Phase-Lock Loops (PLLs) and the various dividers, reset, and feedback
signals associated with the PLLs. This feature gives the user the ability to synthesize clock frequencies and gener-
ate multiple clock signals for routing within the device. Furthermore, it can generate clock signals that are aligned
either at the board level or the device level.
The ispXPGA devices provide up to eight PLLs. Each PLL receives its input clock from its associated global clock
pin, and its output is routed to the associated global clock net. For example, PLL0 receives its clock input from the
GCLK0 global clock pin and provides output to the CLK0 global clock net. The PLL also has the ability to output a
secondary clock that is a division of the primary clock output. When using the secondary clock, the secondary
clock will be routed to the neighboring global clock net. For example, PLL0 will drive its primary clock output on the
CLK0 global clock net and its secondary clock output will drive the CLK1 global clock net. Additionally, each PLL
has a set of PLL_RST, PLL_FBK, and PLL_LOCK signals. The PLL_RST signal can be generated through routing
or a dedicated dual-function I/O pin. The PLL_FBK signal can be generated through a dedicated dual-function I/O
pin or internally from the Global Clock net associated with the PLL. The PLL_LOCK signal feeds routing directly
from the sysCLOCK PLL circuit. Figure 17 illustrates how the PLL_RST and PLL_FBK signals are generated.
Each PLL has four dividers associated with it, M, N, V, and K. The M divider is used to divide the clock signal, while
the N divider is used to multiply the clock signal. The V divider allows the VCO frequency to operate at higher fre-
quencies than the clock output, thereby increasing the frequency range. The K divider is only used when a sec-
ondary clock output is needed. This divider divides the primary clock output and feeds to the adjacent global clock
net. Different combinations of these dividers allow the user to synthesize clock frequencies. Figure 16 shows the
ispXPGA PLL block diagram.
The PLL also has a delay feature that allows the output clock to be advanced or delayed to improve set-up and
clock-to-out times for better performance. This operates by inserting delay on the input or feedback lines of the
PLL. For more information on the PLL, please refer to Lattice technical note number TN1003, sysCLOCK PLL
Usage and Design Guidelines, available at www.latticesemi.com.
Figure 16. ispXPGA PLL Block Diagram
GCLK_IN
PLL_RST
Input Clock
(M) Divider
Programable
Delay
PLL
Post-scalar
(V) Divider
CLK_OUT
Clock Net
PLL_LOCK
PLL_FBK
Feedback
Loop
(N) Divider
Secondary
Clock
(K) Divider
SEC_OUT
Clock Net
14