
EMA_CLK
EMA_CS[0]
EMA_WE_DQM
EMA_BA
EMA_A
EMA_D
EMA_RAS
EMA_CAS
EMA_WE
Bank
Row
Col
D1
D2
D3
D4
ACTV
READ
CL=3
D5
D6
D7
D8
Architecture
847
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
External Memory Interface A (EMIFA)
19.2.4.9 SDRAM Read Operation
When the EMIFA 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
EMIFA proceeds to issue a READ command while specifying the desired bank and column address.
EMA_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 EMIFA 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 EMIFA 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 EMIFA will truncate a series of bursting data if the remaining addresses of the burst are not required
to complete the request. The EMIFA 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 19-5. Timing Waveform for Basic SDRAM Read Operation
Several other pins are also active during a read access. The EMA_WE_DQM[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 EMIFA pins during each command can be found in
The EMIFA 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 EMIFA 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.