EMIF Module Architecture
803
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
External Memory Interface (EMIF)
Table 21-11. SDRAM LOAD MODE REGISTER Command
EMIF_A[9:7]
EMIF_A[6:4]
EMIF_A[3]
EMIF_A[2:0]
0 (Write bursts are of
the programmed burst
length in
EMIF_A[2:0])
These bits control the CAS latency of the
SDRAM and are set according to CL field in
the SDRAM configuration register (SDCR)
as follows:
• If CL = 2, EMIF_A[6:4] = 2h
(CAS latency = 2)
• If CL = 3, EMIF_A[6:4] = 3h
(CAS latency = 3)
0 (Sequential Burst
Type. Interleaved
Burst Type not
supported)
These bits control the burst length of the
SDRAM and are set according to the NM
field in the SDRAM configuration register
(SDCR) as follows:
• If NM = 0, EMIF_A[2:0] = 2h
(Burst Length = 4)
• If NM = 1, EMIF_A[2:0] = 3h
(Burst Length = 8)
21.2.5.5 SDRAM Configuration Procedure
There are two different SDRAM configuration procedures. Although EMIF automatically performs the
SDRAM initialization sequence described in
when coming out of reset, it is recommended
to follow one of the procedures listed below before performing any EMIF memory requests. Procedure A
should be followed if it is determined that the SDRAM Power-up constraint was not violated during the
SDRAM Auto-Initialization Sequence detailed in
on coming out of Reset. The SDRAM
Power-up constraint specifies that 200
μ
s (sometimes 100
μ
s) should exist between receiving stable Vdd
and CLK and the issuing of a PRE command. Procedure B should be followed if the SDRAM Power-up
constraint was violated. The 200
μ
s (100
μ
s) SDRAM Power-up constraint will be violated if the frequency
of EMIF_CLK is greater than 50 MHz (100 MHz for 100
μ
s SDRAM power-up constraint) during SDRAM
Auto-Initialization Sequence. Procedure B should be followed if there is any doubt that the Power-up
constraint was not met.
Procedure A
— Following is the procedure to be followed if the SDRAM Power-up constraint was NOT
violated:
1. Place the SDRAM into Self-Refresh Mode by setting the SR bit of SDCR to 1. A byte-write to the upper
byte of SDCR should be used to avoid restarting the SDRAM Auto-Initialization Sequence described in
. The SDRAM should be placed into Self-Refresh mode when changing the frequency
of EMIF_CLK to avoid incurring the 200
μ
s Power-up constraint again.
2. Configure the desired EMIF_CLK clock frequency. The frequency of the memory clock must meet the
timing requirements in the SDRAM manufacturer's documentation and the timing limitations shown in
the electrical specifications of the device datasheet.
3. Remove the SDRAM from Self-Refresh Mode by clearing the SR bit of SDCR to 0. A byte-write to the
upper byte of SDCR should be used to avoid restarting the SDRAM Auto-Initialization Sequence
described in
4. Program SDTIMR and SDSRETR to satisfy the timing requirements for the attached SDRAM device.
The timing parameters should be taken from the SDRAM datasheet.
5. Program the RR field of SDRCR to match that of the attached device's refresh interval. See
details on determining the appropriate value.
6. Program SDCR to match the characteristics of the attached SDRAM device. This will cause the auto-
initialization sequence in
to be re-run. This second initialization generally takes much
less time due to the increased frequency of EMIF_CLK.
Procedure B
— Following is the procedure to be followed if the SDRAM Power-up constraint was
violated:
1. Configure the desired EMIF_CLK clock frequency. The frequency of the memory clock must meet the
timing requirements in the SDRAM manufacturer's documentation and the timing limitations shown in
the electrical specifications of the device datasheet.
2. Program SDTIMR and SDSRETR to satisfy the timing requirements for the attached SDRAM device.
The timing parameters should be taken from the SDRAM datasheet.
3. Program the RR field of SDRCR such that the following equation is satisfied: (RR × 8)/(f
EMIF_CLK
) >
200
μ
s (sometimes 100
μ
s). For example, an EMIF_CLK frequency of 100 MHz would require setting
RR to 2501 (9C5h) or higher to meet a 200
μ
s constraint.