non-blocking read activity during the CRE read or write-enable transfer. During a write to the PSRAM's
CR, the configuration data is written out on data pins [20:0] of the EPI bus. For a PSRAM configuration
read access, the
RDCRE
bit in the
EPIHB16CFG
register is set to signal that the next access is a
read of the PSRAM configuration register (CR). The address for the CR is written to bits
CR[19:18]
of the
EPIHBPSRAM
register. The read data is returned at
CR
bits [15:0] of the
EPIHBPSRAM
register.
Note:
CRE read and write operations may only occur in asynchronous mode. During
synchronous mode the CRE bit should be disabled. Setting the CRE bit during
synchronous PSRAM accesses can lead to unpredictable behavior.
■
■ When the chip select is programmed to access the PSRAM, the
MODE
bit of the
EPIHBnCFGn
register must be programmed to enable address and data muxed
(ADMUX). Page mode accesses are not supported by the EPI.
■ BURST is optimized for word-length bursting for SDRAM and PSRAM accesses.
The subsequent list identifies the steps for initializing the PSRAM interface:
1.
Follow the EPI initialization steps in “Initialization and Configuration” on page 821.
2.
Enable Host Bus 16 Mode by setting the MODE bits in the
EPICFG
register to 0x13. Choose
between an integer or formula clock divide for the baud rate by configuring the
INTDIV
bit in
the
EPICFG
register.
3.
Configure the
EPIBAUD
register to the desired baud rate.
4.
Since the EPI module only supports asynchronous programming of the configuration registers,
clock gate the EPI clock by programming both the
CLKGATE
and
CLKGATEI
bits in the
EPIHB16CFG
register to 0.
5.
Prepare for writing the PSRAM's Bus Configuration Register by setting the
ALEHIGH
= 1 and
MODE
=0x0 in the
EPIHB16CFG
register.
6.
Program the
EPIHBPSRAM
register to be loaded into the CR register of the PSRAM by
configuring bits [21:0].
■ CR[20:19] =0x0, reserved
■ CR[19:18] = 0x2 to enable configuring of the CR register
■ CR[15]= 0x1 to enable asynchronous access
■ CR [14] = 0 if the iRDY signal is used for memory transfers; if the design will not use the
iRDY signal CR[14] should be cleared.
■ CR[13:11] must be programmed to have a matching read and write wait state configuration
as is programmed in the
EPIHB16CFG
and
EPIHB16TIME
register.
■ CR[10] configures the polarity of the WAIT signal and should match the configuration of the
IRDYINV
bit in the
EPIHB16CFG
register.
■ CR[8]=0x1 to configure the appropriate wait configuration of the data
■ CR[2:0]=0x7 since the EPI interface in PSRAM mode is a continuous burst access.
837
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller