Figure 11-4. SDRAM Write Cycle
Row
Column-1
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
Write
Burst
Term
AD [15:0] driven out
AD [15:0] driven out
11.4.3
Host Bus Mode
Host Bus supports the traditional 8-bit and 16-bit interfaces popularized by the 8051 devices and
SRAM devices, as well as PSRAM and NOR Flash memory. This interface is asynchronous and
uses strobe pins to control activity. Addressable memory can be doubled using Host Bus-16 mode
as it performs half-word accesses. The
EPI0S0
is the LSB of the address and is equivalent to the
internal Cortex-M4 A1 address.
EPI0S0
should be connected to A0 of 16-bit memories.
11.4.3.1
Control Pins
The main three strobes are Address Latch Enable (ALE), Write (WRn), and Read (RDn, sometimes
called OEn). Note that the timings are designed for older logic and so are hold-time versus setup-time
specific. The polarity of the read and write strobes can be active High or active Low by clearing or
setting the
RDHIGH
and
WRHIGH
bits in the
EPI Host-Bus n Configuration (EPIHBnCFGn)
register.
The ALE can be changed to an active-low chip select signal, CSn, through the
EPIHBnCFGn
register. The ALE is best used for Host-Bus muxed mode in which EPI address and data pins are
shared. All Host-Bus accesses have an address phase followed by a data phase. The ALE indicates
to an external latch to capture the address then hold it until the data phase. The polarity of the ALE
can be active High or Low by clearing or setting the
ALEHIGH
bit in the
EPI Host-Bus n
Configuration (EPIHBnCFGn)
register. CSn is best used for Host-Bus unmuxed mode in which
EPI address and data pins are separate. The CSn indicates when the address and data phases of
a read or write access are occurring. Both the ALE and the CSn modes can be enhanced to access
four external devices using settings in the
EPIHBnCFGn
register. PSRAM accesses must use both
ALE and CSn . Wait states can be added to the data phase of the access using the
WRWS
and
RDWS
bits in the
EPIHBnCFGn
register. Additionally, within these wait state options, the
WRWSM
and
RDWSM
bit of the
EPIHBnTIMEn
register can be set to reduce the given wait states by 1 EPI clock cycle for
finer granularity.
For FIFO mode, the ALE is not used, and two input holds are optionally supported to gate input and
output to what the XFIFO can handle. FIFO mode is only applicable in EPI asynchronous mode.
Host-Bus 8 and Host-Bus 16 modes are very configurable. The user has the ability to connect 1,2,
or 4 external devices to the EPI signals, as well as control whether byte select signals are provided
in HB16 mode. These capabilities depend on the configuration of the
MODE
field in the
EPIHBnCFG
June 18, 2014
826
Texas Instruments-Production Data
External Peripheral Interface (EPI)