EMIF_CLK
EMIF_nCS[0]
EMIF_nDQM
EMIF_BA
EMIF_A
EMIF_D
EMIF_nRAS
EMIF_nCAS
EMIF_nWE
Bank
Row
Col
D1
D2
D3
D4
ACTV
READ
CL=3
D5
D6
D7
D8
EMIF Module Architecture
807
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
External Memory Interface (EMIF)
21.2.5.9 SDRAM Read Operation
When the EMIF receives a read request to SDRAM from one of the requesters listed in
, it
performs one or more read access cycles. A read access cycle begins with the issuing of the ACTV
command to select the desired bank and row of the SDRAM device. After the row has been opened, the
EMIF proceeds to issue a READ command while specifying the desired bank and column address.
EMIF_A[10] is held low during the READ command to avoid auto-precharging. The READ command
signals the SDRAM device to start bursting data from the specified address while the EMIF issues NOP
commands. Following a READ command, the CL field of the SDRAM configuration register (SDCR)
defines how many delay cycles will be present before the read data appears on the data bus. This is
referred to as the CAS latency.
shows the signal waveforms for a basic SDRAM read operation in which a burst of data is
read from a single page. When the EMIF SDRAM interface is configured to 16 bit by setting the NM bit of
the SDRAM configuration register (SDCR) to 1, a burst size of eight is used.
shows a burst
size of eight.
The EMIF will truncate a series of bursting data if the remaining addresses of the burst are not required to
complete the request. The EMIF can truncate the burst in three ways:
•
By issuing another READ to the same page in the same bank.
•
By issuing a PRE command in order to prepare for accessing a different page of the same bank.
•
By issuing a BT command in order to prepare for accessing a page in a different bank.
Figure 21-5. Timing Waveform for Basic SDRAM Read Operation
Several other pins are also active during a read access. The EMIF_nDQM[1:0] pins are driven low during
the READ commands and are kept low during the NOP commands that correspond to the burst request.
The state of the other EMIF pins during each command can be found in
.
The EMIF schedules its commands based on the timing information that is provided to it in the SDRAM
timing register (SDTIMR). The values for the timing parameters in this register should be chosen to satisfy
the timing requirements listed in the SDRAM datasheet. The EMIF uses this timing information to avoid
violating any timing constraints related to issuing commands. This is commonly accomplished by inserting
NOP commands between various commands during an access. Refer to the register description of
SDTIMR in
for more details on the various timing parameters.