Chapter 9. SDRAM Controller
9-9
SDRAM Registers
9.5.2 SDRAM Timing Register (SDTR)
The SDTR is used to configure SDRAM controller refresh counters for the type of SDRAM
devices used and the number of clocks required for each type of SDRAM access. The reset
value is 0x2115. For lower CPU clock frequencies, precharge and activate times can be
reduced to eliminate up to 2 clock cycles from the read and write accesses. Consult the data
sheets of the SDRAMs being considered.
Table 9-8 describes SDTR fields.
15
10
9
8
7
6
5
4
3
2
1
0
Write
RTP
RC
—
RP
RCD
CLT
Reset
0010_00
01
00
01
01
01
R/W
R/W
Addr
MBAR + 0x0184
Figure 9-4. SDRAM Timing Register (SDTR)
Table 9-8. SDTR Field Descriptions
Bits
Name
Description
15–10
RTP
Refresh timer prescaler. Determines the number of clock cycles x 16 between refreshes. The
following table describes different recommended prescaler settings for different clock frequencies
including a margin of 1.2
µ
S. Recommended values are as follows:
RTP
15.6 µs = 1/f*RTP*16
System Clock
111101
61
66 MHz
101011
43
48 MHz
011101
29
33 MHz
010110
22
25 MHz
000100
4
5 MHz (emulator)
9–8
RC
Refresh count. Indicates the number of clock cycles spent in refresh state (RC + 5). Refresh occurs
during the first of these clock cycles; the rest of the time is the delay that must occur before the
SDRAM is ready to do anything else.
00 5 cycles
01 6 cycles (default)
10 7 cycles
11 8 cycles
7–6
—
Reserved, should be cleared.
5–4
RP
Precharge time. Specifies number of clock cycles taken for a precharge (RP + 1).
00 1 cycle
01 2 cycles (default)
10 3 cycles
11 4 cycles
Содержание DigitalDNA ColdFire MCF5272
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