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Row
Column
Data 0
Data 1
...
Data n
CLK
(EPI0S31)
CKE
(EPI0S30)
CSn
(EPI0S29)
WEn
(EPI0S28)
RASn
(EPI0S19)
CASn
(EPI0S18)
DQMH, DQML
(EPI0S [17:16])
AD [15:0]
(EPI0S
[15:0])
Activate
NOP
Read
NOP
Burst
Term
AD [15:0] driven in
AD [15:0] driven out
AD [15:0] driven out
Initialization and Configuration
1093
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
External Peripheral Interface (EPI)
16.4.2.2 Refresh Configuration
The refresh count is based on the external clock speed and the number of rows per bank as well as the
refresh period. The RFSH field represents how many external clock cycles remain before an AUTO-
REFRESH is required. The normal formula is:
RFSH
≤
(t
Refresh_us
/ number_rows) / ext_clock_period_µs
A refresh period is normally 64 ms, or 64000
μ
s. The number of rows is normally 4096 or 8192. The
ext_clock_period is a value expressed in µs and is derived by dividing 1000 by the clock speed expressed
in MHz. So, 50 MHz is 1000 / 50 = 20 ns, or 0.02
μ
s. A typical SDRAM is 4096 rows per bank if the
system clock is running at 50 MHz with an EPIBAUD register value of 0:
RFSH = (64000 / 4096) / 0.02 = 15.625
μ
s / 0.02
μ
s = 781.25.
The default value in the RFSH field is 750 decimal or 0x2EE to allow for a margin of safety and providing
15
μ
s per refresh. It is important to note that this number should always be smaller or equal to what is
required by the above equation. For example, if running the external clock at 25 MHz (40 ns per clock
period), 390 is the highest number that may be used. The external clock may be 25 MHz when the system
clock is 25 MHz or when the system clock is 50 MHz and configuring the COUNT0 field in the EPIBAUD
register to 1 (divide by 2).
If a number larger than allowed is used, the SDRAM is not refreshed often enough, and data is lost.
16.4.2.3 Bus Interface Speed
The EPI Controller SDRAM interface can operate up to 60 MHz. The COUNT0 field in the EPIBAUD
register configures the speed of the EPI clock. For system clock (SysClk) speeds up to 60 MHz, the
COUNT0 field can be 0x0000, and the SDRAM interface can run at the same speed as SysClk. However,
if SysClk is running at higher speeds, the bus interface can run only as fast as half speed, and the
COUNT0 field must be configured to at least 0x0001.
16.4.2.4 Nonblocking Read Cycle
shows a nonblocking read cycle of n halfwords; n can be any number greater than or equal to
1. The cycle begins with the Activate command and the row address on the EPI0S[15:0] signals. With the
programmed CAS latency of 2, the Read command with the column address on the EPI0S[15:0] signals
follows after 2 clock cycles. Following one more NOP cycle, data is read in on the EPI0S[15:0] signals on
every rising clock edge. The Burst Terminate command is issued during the cycle when the next-to-last
halfword is read in. The DQMH and DQML signals are deasserted after the last halfword of data is
received; the CSn signal deasserts on the following clock cycle, signaling the end of the read cycle. At
least one clock period of inactivity separates any two SDRAM cycles.
Figure 16-2. SDRAM Nonblocking Read Cycle