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Architecture
859
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
External Memory Interface A (EMIFA)
19.2.5.5 Read and Write Operation in Select Strobe Mode
Select Strobe Mode is the EMIFA'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 EMA_WE_DQM pins
operate as byte enables and the EMA_CS[n] (n = 2, 3, 4, or 5) 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.
19.2.5.5.1 Asynchronous Read Operations (Select Strobe Mode)
NOTE:
During the entirety of an asynchronous read operation, the EMA_WE 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 EMIFA'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 EMIFA 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 19-21. 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 EMIFA, the EMIFA 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 to the same chip select, no
turn-around cycles are inserted.
After the EMIFA 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 EMIFA proceeds to the setup period of the operation. If it is no
longer the highest priority task, the EMIFA 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 EMA_A and EMA_BA become valid and carry the values described in
• The EMA_WE_DQM pins become valid as byte enables.
Strobe period
The following actions occur during the strobe period of a read operation:
1.
EMA_CS[n] (n = 2, 3, 4, or 5) and EMA_OE 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:
•
EMA_CS[n] (n = 2, 3, 4, or 5) and EMA_OE rise
•
The data on the EMA_D bus is sampled by the EMIFA.
In
, EMA_WAIT is inactive. If EMA_WAIT 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
EMA_WAIT pin.
End of the hold
period
At the end of the hold period:
• The address pins EMA_A and EMA_BA become invalid
• The EMA_EMA_WE_DQM pins become invalid
The EMIFA 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 EMIFA 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 EMIFA 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 EMIFA
instead enters directly into the turnaround period for the pending read or write operation.