Chapter 3: Memory Blocks in the Cyclone III Device Family
3–15
Design Considerations
Read or Write Clock Mode
Cyclone III device family M9K memory blocks can implement read or write clock
mode for FIFO and simple dual-port memories. In this mode, a write clock controls
the data inputs, write address, and wrenregisters. Similarly, a read clock controls the
data outputs, read address, and rdenregisters. M9K memory blocks support
independent clock enables for both the read and write clocks.
When using read or write mode, if you perform a simultaneous read or write to the
same address location, the output read data is unknown. If you require the output
data to be a known value, use either single-clock mode, input clock mode, or output
clock mode and choose the appropriate read-during-write behavior in the
MegaWizard Plug-In Manager.
Single-Clock Mode
Cyclone III device family M9K memory blocks can implement single-clock mode for
FIFO, ROM, true dual-port, simple dual-port, and single-port memories. In this mode,
you can control all registers of the M9K memory block with a single clock together
with clock enable.
Design Considerations
This section describes designing with M9K memory blocks.
Read-During-Write Operations
“Same-Port Read-During-Write Mode” on page 3–16 and “Mixed-Port Read-During-
Write Mode” on page 3–16 describe the functionality of the various RAM
configurations when reading from an address during a write operation at that same
address.
There are two read-during-write data flows: same-port and mixed-port. Figure 3–14
shows the difference between these flows.
Figure 3–14. Cyclone III Device Family Read-During-Write Data Flow
write_a
write_b
Port A
data in
Port B
data in
Mixed-port
data flow
Same-port
data flow
read_a
Port A
Port B
read_b
data out
data out
December 2011 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
