Initialization and Configuration
1091
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
External Peripheral Interface (EPI)
0xA000.0000 or 0x2 for address 0xC000.0000; and program the EPSZ field to 0x0 for 256 bytes.
8. To read or write directly, use the mapped address area (configured with the EPIADDRMAP register).
Up to 4 or 5 writes can be performed at once without blocking. Each read is blocked until the value is
retrieved.
9. To perform a nonblocking read, see
.
NOTE:
The application should not attempt external access until eight system clock cycles after the
EPI has been fully configured.
NOTE:
When a MODE field has been programmed in the EPICFG register, the application should
reset all configuration registers before programming to a new MODE value.
The following subsections describe the initialization and configuration for each of the modes of operation.
Initialize everything properly to ensure correct operation. Control of the GPIO states is also important, as
changes may cause the external device to interpret pin states as actions or commands (see ). Normally, a
pullup or pulldown is needed on the board to at least control the chip-select or chip-enable as the GPIOs
come out of reset in high-impedance.
16.4.1 EPI Interface Options
There are a variety of memories and peripherals that can interface to the EPI module.
shows
the various configurations with their maximum performance.
Table 16-1. EPI Interface Options
Interface
Maximum Frequency
Single SDRAM
60 MHz
Single SRAM
60 MHz
Single PSRAM without iRDY signal use
55 MHz
Single PSRAM with iRDY signal use
52 MHz
FPGAs, CPLDs, and others using general-purpose mode
60 MHz
Memory configurations with 2 chip selects
40 MHz
Memory configurations with 4 chip selects
20 MHz
16.4.2 SDRAM Mode
When activating the SDRAM mode, it is important to consider a few points:
•
Generally, it takes over 100
μ
s 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.
•
The INITSEQ bit in the EPI Status (EPISTAT) register can be checked to determine when the
initialization sequence is complete.
•
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.
•
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.
•
Before entering SLEEP mode, make sure all nonblocking reads and normal reads and writes have