Memory Interfaces
4-29
Memory Interface Traffic Controller
The following formula is used to determine the refresh counter value that
must be programmed in the EMIF fast interface configuration register 1
(EMIFF_SDRAM_CONFIG):
Counter Value
+
ǒ
SDRAM refresh rate
T
f
Ǔ
–400
Number of SDRAM Rows
where T
f
= (1 / traffic controller frequency) and the 400 cycles take into
account the worst-case priority scenario where the SDRAM refresh is at
the bottom of the priority queue.
Example: 64-ms refresh rate, 100-MHz traffic controller frequency,
4096 rows to be refreshed:
T
f
= 10 ns
Counter Value
+
ǒ
64000000 ns
10 ns
Ǔ
–400
4096
+
1562 cycles
This ensures a 64-millisecond refresh period for the full SDRAM. In the event
that the downcounter does not equal zero by the time a new autorefresh burst
request occurs, the new request is memorized and is done during the current
refresh burst.
4.3.3.5
SDRAM Self-Refresh Protection
The traffic controller idle mode is entered after an internal request and
acknowledge protocol with the OMAP5910 clock generator and system reset
module. In idle mode, the traffic controller clock is stopped. If the clock remains
idle for more than 64 milliseconds and the SDRAM was not entered into self
refresh mode, SDRAM data corruption results. Setting the RFRSH_STBY
bit in the EMIF fast interface SDRAM configuration register 2
(EMIFF_SDRAM_CONFIG_2) avoids SDRAM data corruption by automati-
cally placing the SDRAM in self-refresh mode prior to the traffic controller
entering idle mode.
A similar SDRAM data corruption can occur in the event of a warm global sys-
tem reset from external device pin. Since the reset event is likely to extend be-
yond 64 milliseconds, and the SDRAM controller does not autorefresh during
reset, data is corrupted. Setting the RFRSH_RST bit in the EMIF fast interface
SDRAM configuration register 2 (EMIFF_SDRAM_CONFIG_2) avoids
SDRAM data corruption for this case by automatically placing the SDRAM in
self-refresh mode prior to warm reset being applied to the traffic controller. The
SDRAM controller continues in self-refresh mode until the reset is unasserted.
Note that RFRSH_RST applies only in the case of warm reset. For cold reset,
SDRAM is not set to self-refresh regardless of the state of RFRSH_RST.