v2.0
General-Purpose SDRAM Controller
SD R A M Co n t r o l l er Fu nc t i o na l
D es cr i p t i o n
The general-purpose SDRAM controller is designed to
provide simplified control of many different sizes of
SDRAMs. The controller architecture provides control for
data bursts by linearly incrementing the address. The user
starts a burst at a specified address and the burst continues
until the user terminates it.
SD R A M Co n t r o l l er Si g na l s
CLK
RESET_N
ACTIVATE
WR_CYC
ADDR
CYC_DONE
WR_BE_DONE
RD_BE_DONE
RD_BE_RDY
WR_BE_RDY
MADDR
CSn
RASn
CASn
WEn
BA
DQM
CKE
The SDRAM controller communicates with a user鈥檚
functions and drives the control signals into the
SDRAM. When the controller recognizes the start of a write
cycle, the controller prepares the SDRAM to accept data
and then indicates readiness by driving the WR_BE_RDY
signal. When the controller recognizes the start of a read
cycle, the controller prepares the SDRAM to provide data
and then indicates readiness by driving the RD_BE_RDY
signal.
The WR_BE_RDY and RD_BE_RDY signals are one-stage
pipelined. On write cycles, the WR_BE_RDY signal is
asserted one clock cycle prior to the time when data can
actually be accepted by the SDRAM. On read cycles, the
RD_BE_RDY signal is asserted one clock cycle prior to the
time when SDRAM data is valid. In general, the *_RDY
signals indicate that the SDRAM and controller will ready
for data transfer on the next cycle.
Table 1 鈥?/div>
SDRAM Controller Signals
Name
Inputs from the User鈥檚 System
Clk
RESETn
ACTIVATE
WR_CYC
ADDR(M:0)
CYC_DONE
WR_BE_NOW
RD_BE_NOW
Figure 1 鈥?/div>
SDRAM Controller
The user provides a WR_BE_NOW and RD_BE_NOW signal
to the controller. When WR_BE_RDY and WR_BE_NOW
are both asserted at the same time, or when RD_BE_RDY
and RD_BE_NOW are both asserted at the same time, data
is transferred. If the WR_BE_NOW signal is not asserted,
the controller will retain its current address until the
WR_BE_NOW signal asserts at which time data is
transferred. Because of the pipelined nature of the SDRAM
and the assumption that burst addresses are incremented
linearly, a deassertion of RD_BE_NOW causes delay while
the controller backs up and refills the pipeline.
At the conclusion of a cycle, the user asserts the
CYCLE_DONE signal for one cycle.
Description
This is the system clock. The controller can transfer data at this rate during bursts.
This is a reset signal. Assertion of this signal causes the SDRAM to be initialized.
A pulse on this signal causes the Controller to prepare the SDRAM for a cycle.
This signal defines the type of cycle started when the ACTIVE
(ACTIVATE?)
signal is asserted.
This is the beginning address of the burst.
This signal causes the cycle to be terminated.
This signal indicates that the user wants the currently valid data to be written.
This signal indicates that the user will accept the data on this cycle.
Outputs to the User鈥檚 System
F eb r u a r y 2 0 0 0
1
漏 2000 Actel Corporation
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