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Global Resources in Actel Low-Power Flash Devices
3 – Global Resources in Actel Low-Power Flash
Devices
Introduction
Actel IGLOO, Fusion, and ProASIC3 FPGA devices offer a powerful, low-delay VersaNet global
network scheme and have extensive support for multiple clock domains. In addition to the Clock
Conditioning Circuits (CCCs) and phase-locked loops (PLLs), there is a comprehensive global clock
distribution network called a VersaNet global network. Each logical element (VersaTile) input and
output port has access to these global networks. The VersaNet global networks can be used to
distribute low-skew clock signals or high-fanout nets. In addition, these highly segmented VersaNet
global networks offer users the flexibility to create low-skew local networks using spines. This
document describes VersaNet global networks and discusses how to assign signals to these global
networks and spines in a design flow. Details concerning low-power flash device PLLs are described
low-power flash devices’ global architecture and uses of these global networks in designs.
Global Architecture
Low-power flash devices offer powerful and flexible control of circuit timing through the use of
analog circuitry. Each chip has up to six CCCs, some with PLLs.
In IGLOOe, ProASIC3EL, and ProASIC3E devices, all CCCs have PLLs—hence, 6 PLLs per device.
In IGLOO, IGLOO PLUS, ProASIC3L, and ProASIC3 devices, the west CCC contains a PLL core
(except in 15 k and 30 k devices).
Each PLL includes delay lines, a phase shifter (0°, 90°, 180°, 270°), and clock multipliers/dividers.
Each CCC has all the circuitry needed for the selection and interconnection of inputs to the
VersaNet global network. The east and west CCCs each have access to three VersaNet global lines
on each side of the chip (six global lines total). The CCCs at the four corners each have access to
three quadrant global lines in each quadrant of the chip (except in 15 k gate and 30 k gate
devices).
In 15 k and 30 k gate devices, all six VersaNet global lines are driven from three southern I/Os,
located toward the east and west sides. Each of these tiles can be configured to select a central I/O
on its respective side or an internal routed signal as the input signal. 15 k and 30 k gate devices do
not support any clock conditioning circuitry, nor do they contain the VersaNet global network
concept of top and bottom spines.
The flexible use of the VersaNet global network allows the designer to address several design
requirements. User applications that are clock-resource-intensive can easily route external or gated
internal clocks using VersaNet global routing networks. Designers can also drastically reduce delay
penalties and minimize resource usage by mapping critical, high-fanout nets to the VersaNet global
network.
The following sections give an overview of the VersaNet global network, the structure of the
global network, and the clock aggregation feature that enables a design to have very low clock
skew using spines.