External Memory Controller
LH79524/LH79525 User’s Guide
7-56
Version 1.0
7.5.2.24 Static Memory Read Delay Registers (SWAITRDx)
The Static Memory Read Delay Registers enable programming Read cycle wait states. A
complete description of programming this register appears in Section 7.2.4.1.1.
The total Read cycle time is the total time that the address is valid. During this time, several
parameters are programmable. In the following equations, ‘D’ represents the SWAITOENx
register, ‘E’ represents the SWAITRDx register, and ‘C’ represents the address hold time.
In general, Read wait states can be derived from the following equation:
tRC (Read cycle time) = tD1 + tD2 + ... tDn + tE0 + tE1 + ... tEn + C, where the length of
each term is one HCLK period, and ‘n’ is the value programmed in the respective register.
The minimum value for the equation is tRC = tE0 + C, and is therefore the zero wait
state timing.
Thus, Read wait states can be programmed using the appropriate mix of nOE extension
(programmed in SWAITRDx) with nOE assertion delay (programmed in SWAITOENx).
These registers must only be modified during system initialization, or when there are no
current or outstanding transactions. Software can ensure that there are no current or out-
standing transactions by waiting until the memory controller is idle, then entering Low-
Power Mode (CONTROL:MODE = 1), or Disable Mode (CONTROL:ENABLE = 0). When
in these two modes, external memory access is not allowed, ensuring that changing
parameters will not corrupt external data. Low-Power Mode automatically refreshes
SDRAM; Disable Mode requires commanding the SDRAM to Self Refresh
(DYNMCTRL:SR = 1) prior to entering Disable.
Table 7-58. SWAITRDx Register
BIT
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
FIELD
///
RESET
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
TYPE
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
BIT
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
FIELD
///
WAITRD
RESET
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
TYPE
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RW
RW
RW
RW
RW
ADDR
0xFF 0x20C for SWAITRD0
0xFF 0x22C for SWAITRD1
0xFF 0x24C for SWAITRD2
0xFF 0x26C for SWAITRD3
Table 7-59. SWAITRDx Fields
BITS
NAME
FUNCTION
31:5
///
Reserved
Reading returns 0. Write the reset value.
4:0
WAITRD
Read Access Delay
Non-page mode Read or Asynchronous Page Mode
Read, delay first read only:
Delay = (n + 1) HCLK cycles