TM4C1294NCPDT Datasheet, PDF (822/1890 Pages) Texas Instruments – Tiva Microcontroller

External Peripheral Interface (EPI)
11.4.1
11.4.2
EPI Interface Options
There are a variety of memories and peripherals that can interface to the EPI module. Table
11-2 on page 822 shows the various configurations with their maximum performance.
Table 11-2. EPI Interface Options
Interface
Single SDRAM
Single SRAM
Single PSRAM without iRDY signal use
Single PSRAM with iRDY signal use
FPGAs, CPLDs, etc using General Purpose Mode
Memory configurations with 2 chip selects
Memory configurations with 4 chip selects
Maximum Frequency
60 MHz
60 MHz
55 MHz
52 MHz
60 MHz
40 MHz
20 MHz
SDRAM Mode
When activating the SDRAM mode, it is important to consider a few points:
1. Generally, it takes over 100 μs from when the mode is activated to when the first operation is
allowed. The SDRAM controller begins the SDRAM initialization sequence as soon as the mode
is selected and enabled via the EPICFG register. It is important that the GPIOs are properly
configured before the SDRAM mode is enabled, as the EPI controller is relying on the GPIO
block's ability to drive the pins immediately. As part of the initialization sequence, the LOAD
MODE REGISTER command is automatically sent to the SDRAM with a value of 0x27, which
sets a CAS latency of 2 and a full page burst length.
2. The INITSEQ bit in the EPI Status (EPISTAT) register can be checked to determine when the
initialization sequence is complete.
3. When using a frequency range and/or refresh value other than the default value, it is important
to configure the FREQ and RFSH fields in the EPI SDRAM Configuration (EPISDRAMCFG)
register shortly after activating the mode. After the 100-μs startup time, the EPI block must be
configured properly to keep the SDRAM contents stable.
4. The SLEEP bit in the EPISDRAMCFG register may be configured to put the SDRAM into a
low-power self-refreshing state. It is important to note that the SDRAM mode must not be
disabled once enabled, or else the SDRAM is no longer clocked and the contents are lost.
5. Before entering SLEEP mode, make sure all non-blocking reads and normal reads and writes
have completed. If the system is running at 30 to 50 MHz, wait 2 EPI clocks after clearing the
SLEEP bit before executing non-blocking reads, or normal reads and writes. If the system is
configured to greater than 50 MHz, wait 5 EPI clocks before read and write transactions. For
all other configurations, wait 1 EPI clock.
The SIZE field of the EPISDRAMCFG register must be configured correctly based on the amount
of SDRAM in the system.
The FREQ field must be configured according to the value that represents the range being used.
Based on the range selected, the number of external clocks used between certain operations (for
example, PRECHARGE or ACTIVATE) is determined. If a higher frequency is given than is used,
then the only downside is that the peripheral is slower (uses more cycles for these delays). If a lower
frequency is given, incorrect operation occurs.
822
June 18, 2014
Texas Instruments-Production Data