Apple iMac - Manual (Page 50)

C H A P T E R 4
RAM Expansion
50 Main RAM Expansion
The JEDEC specification for the SO-DIMM defines a Serial Presence Detect
(SPD) feature that contains the attributes of the module. SO-DIMMs for use in
the iMac are required to have the SPD feature. Information about the required
values to be stored in the presence detect EEPROM is in section 4.1.2.5 and
Figure 4.5.6–C (144 Pin SDRAM SO–DIMM, PD INFORMATION) of the JEDEC
standard 21-C specification, release 7.
Because the SO-DIMM connector has only two clock lines, and each clock line is
limited to only 4 loads, an SO-DIMM with more than 8 SDRAM devices must
have buffers on the clock lines. The buffers must be zero-delay type, such as
phase-lock loop (PLL), which regenerates the clock signals. For example, the
computer can support a 128-MB SO-DIMM using 16 devices and a PLL clock
buffer.
SDRAM Devices 4
The SDRAM devices used in the RAM expansion modules must be self-refresh
type devices for operation from a 3.3-V power supply. The speed of the SDRAM
devices must be 100 MHz or greater, corresponding to a cycle time of 10 ns or
less.
The devices are programmed to operate with a CAS latency of 3. At that CAS
latency, the access time from the clock transition must be 7 ns or less. The burst
length must be at least 4 and the minimum clock delay for back-to-back random
column access cycles must be a latency of 1 clock cycle.