DRAM Controller
MOTOROLA
MC68322 USER’S MANUAL
7-5
7.2 DRAM CONTROL REGISTER
The DRAM control register (DRMCR) contains two fields—DRAM timing select (TS) and
DRAM refresh interval (RIC). Figure 7-5 illustrates this register.
Figure 7-5. DRAM Control Register
The TS field is used to select one of three DRAM timing modes. The timing mode selected
applies to all banks. All DRAM devices connected to the MC68322 must operate at the same
speed because the DRAM controller does not support independently programmable bank
speeds. The exception to this is the ability to slow down individual DRAM banks by setting
the ROM mode bit in the corresponding DRAM register. See Section 7.1.2 ROM Mode for
details.
The RIC field provides the refresh time interval in CLK1s. The CAS and RAS timing
parameters for refresh are hardcoded and cannot be programmed. However, the RIC field
can be changed at any time and the new value will take effect when the next terminal count
is reached and a refresh cycle begins. Then the new RIC field value is loaded into the refresh
counter.
7.3 DRAM TIMING MODES
Three DRAM timing modes provide wait state profiles optimized for various DRAM speeds
at system frequencies of 16, 20, and 25MHz. These timing modes are not fixed to any
specific system clock frequency and can therefore be used regardless of system clock
frequency as long as the DRAM device timing is satisfied. The DRAM controller
automatically bursts for both read and write bus cycles. Table 7-2 lists the timing modes,
their associated DRAM control register TS field encodings, and the recommended system
speeds.
Table 7-2. DRAM Timing Modes
MODE/WAIT STATE PROFILE
TS FIELD
ENCODING
RECOMMENDED SYSTEM SPEED
Timing Mode 0
5:3:3:3…reads
5:2:2:2…writes
00
25MHz with 80ns
Timing Mode 1
4:2:2:2…reads
4:2:2:2…writes
01
20MHz with 100ns or 25MHz with 70ns
Timing Mode 2
3:2:1:2…reads
3:2:2:2…writes
10
16MHz with 80ns, 20MHz with 60ns, or 25MHz
with 50ns
Not Used
11
Not Used
00FFF160
00FFF162
= RESERVED
TS
REFRESH INTERVAL COUNT (RIC)
15
14
13
12
11
10
9
8
7
65432
1
0
15
14
13
12
11
10
9
8
7
65432
1
0
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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