APEX 20K Programmable Logic Device Family Data Sheet
ESBs can implement synchronous RAM, which is easier to use than
asynchronous RAM. A circuit using asynchronous RAM must generate
the RAM write enable (WE) signal, while ensuring that its data and address
signals meet setup and hold time specifications relative to the WEsignal.
In contrast, the ESB’s synchronous RAM generates its own WEsignal and
is self-timed with respect to the global clock. Circuits using the ESB’s self-
timed RAM must only meet the setup and hold time specifications of the
global clock.
ESB inputs are driven by the adjacent local interconnect, which in turn can
be driven by the MegaLAB or FastTrack Interconnect. Because the ESB can
be driven by the local interconnect, an adjacent LE can drive it directly for
fast memory access. ESB outputs drive the MegaLAB and FastTrack
Interconnect. In addition, ten ESB outputs, nine of which are unique
output lines, drive the local interconnect for fast connection to adjacent
LEs or for fast feedback product-term logic.
When implementing memory, each ESB can be configured in any of the
following sizes: 128 × 16, 256 × 8, 512 × 4, 1,024 × 2, or 2,048 × 1. By
combining multiple ESBs, the Quartus II software implements larger
memory blocks automatically. For example, two 128 × 16 RAM blocks can
be combined to form a 128 × 32 RAM block, and two 512 × 4 RAM blocks
can be combined to form a 512 × 8 RAM block. Memory performance does
not degrade for memory blocks up to 2,048 words deep. Each ESB can
implement a 2,048-word-deep memory; the ESBs are used in parallel,
eliminating the need for any external control logic and its associated
delays.
To create a high-speed memory block that is more than 2,048 words deep,
ESBs drive tri-state lines. Each tri-state line connects all ESBs in a column
of MegaLAB structures, and drives the MegaLAB interconnect and row
and column FastTrack Interconnect throughout the column. Each ESB
incorporates a programmable decoder to activate the tri-state driver
appropriately. For instance, to implement 8,192-word-deep memory, four
ESBs are used. Eleven address lines drive the ESB memory, and two more
drive the tri-state decoder. Depending on which 2,048-word memory
page is selected, the appropriate ESB driver is turned on, driving the
output to the tri-state line. The Quartus II software automatically
combines ESBs with tri-state lines to form deeper memory blocks. The
internal tri-state control logic is designed to avoid internal contention and
floating lines. See Figure 18.
30
Altera Corporation
ETC [ ETC ]
