EMIF Module Architecture
640
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
External Memory Interface (EMIF)
17.2.6.5 Read and Write Operation in Select Strobe Mode
Select Strobe Mode is the EMIF's second mode of operation. It is selected when the SS bit of the
asynchronous
n
configuration register (CE
n
CFG) is set to 1. In this mode, the EMIF_nDQM pins operate
as byte enables and the EMIF_nCS[n] (n = 2, 3, or 4) pin is only active during the strobe period of an
access cycle.
and
explain the details of read and write operations
while in Select Strobe Mode.
17.2.6.5.1 Asynchronous Read Operations (Select Strobe Mode)
NOTE:
During the entirety of an asynchronous read operation, the EMIF_nWE pin is driven high.
An asynchronous read is performed when any of the requesters mentioned in
request a
read from the attached asynchronous memory. After the request is received, a read operation is initiated
once it becomes the EMIF's highest priority task, according to the priority scheme detailed in
. In the event that the read request cannot be serviced by a single access cycle to the
external device, multiple access cycles will be performed by the EMIF until the entire request is fulfilled.
The details of an asynchronous read operation in Select Strobe Mode are described in
. Also,
shows an example timing diagram of a basic read operation.
Table 17-21. Asynchronous Read Operation in Select Strobe Mode
Time Interval
Pin Activity in Select Strobe Mode
Turnaround
period
Once the read operation becomes the highest priority task for the EMIF, the EMIF waits for the programmed
number of turn-around cycles before proceeding to the setup period of the operation. The number of wait cycles is
taken directly from the TA field of the asynchronous
n
configuration register (CE
n
CFG). There are two exceptions
to this rule:
• If the current read operation was directly proceeded by another read operation, no turn-around cycles are
inserted.
• If the current read operation was directly proceeded by a write operation and the TA field has been cleared
to 0, one turn-around cycle will be inserted.
After the EMIF has waited for the turn-around cycles to complete, it again checks to make sure that the read
operation is still its highest priority task. If so, the EMIF proceeds to the setup period of the operation. If it is no
longer the highest priority task, the EMIF terminates the operation.
Start of the
setup period
The following actions occur at the start of the setup period:
• The setup, strobe, and hold values are set according to the R_SETUP, R_STROBE, and R_HOLD values in
CE
n
CFG.
• The address pins EMIF_A and EMIF_BA become valid and carry the values described in
• The EMIF_nDQM pins become valid as byte enables.
Strobe period
The following actions occur during the strobe period of a read operation:
1.
EMIF_nCS[n] (n = 2, 3, or 4) and EMIF_nOE fall at the start of the strobe period
2.
On the rising edge of the clock which is concurrent with the end of the strobe period:
•
EMIF_nCS[n] (n = 2, 3, or 4) and EMIF_nOE rise
•
The data on the EMIF_D bus is sampled by the EMIF.
In
, EMIF_nWAIT is inactive. If EMIF_nWAIT is instead activated, the strobe period can be extended
by the external device to give it more time to provide the data.
contains more details on using the
EMIF_nWAIT pin.
End of the hold
period
At the end of the hold period:
• The address pins EMIF_A and EMIF_BA become invalid
• The EMIF_nDQM pins become invalid
The EMIF may be required to issue additional read operations to a device with a small data bus width in order to
complete an entire word access. In this case, the EMIF immediately re-enters the setup period to begin another
operation without incurring the turnaround cycle delay. The setup, strobe, and hold values are not updated in this
case. If the entire word access has been completed, the EMIF returns to its previous state unless another
asynchronous request has been submitted and is currently the highest priority task. If this is the case, the EMIF
instead enters directly into the turnaround period for the pending read or write operation.