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RAM Expansion

This chapter tells how to gain access to the RAM expansion slot in the iBook and then describes the RAM expansion module.

The RAM Expansion Slot

The iBook has one RAM expansion slot. To get access to the RAM expansion slot, the user must open up the computer. Below is an overview of the steps for opening the computer; for complete instructions, refer to your iBook User Guide.

Shut down the computer.
Unplug the AC adapter from the computer.
Remove the battery from the computer.
Release the two plastic tabs located at the top of the keyboard and pull only on the latches (not on the keys) to lift the keyboard up and turn it over, without disconnecting the keyboard’s membrane cable. See Figure 3-5.
Remove the four screws that secure the metal RAM shield plate and slide the plate toward the rear of the computer, then lift it up and out of the computer. See Figure 4-1.
If an SO-DIMM is currently occupying the memory expansion slot, remove it.

Once the RAM expansion module has been installed in the slot, close up the computer by performing these steps in reverse order.

If AC and battery power are removed for longer than ten minutes, the user may need to reset the computer’s clock (using the Data and Time pane in Systems Preferences) when the computer is turned back on.

The RAM Expansion Module

The RAM expansion slot accommodates a standard DDR266B (PC2100) SO-DIMM (small outline, dual inline memory module) that uses SDRAM devices.

An SO-DIMM for the iBook can contain either 128, 256, 512 MB, or 1 GB of memory. Maximum supported RAM capacity is 1.5 GB and is limited by the space available for the SO-DIMM.

Mechanical Design of the RAM SO-DIMM

The mechanical characteristics of the RAM expansion module are given in the JEDEC specification JESD95 for the 200-pin 8-byte DRAM SO-DIMM. To find out how to obtain the specification, see RAM Expansion Modules.

The specification defines SO-DIMMs with nominal heights of 1.0, 1.25, 1.5, or 2.0 inches. The iBook can accommodate standard SO-DIMMs with a height of 1.0 or 1.25 inches.

The JEDEC specification defines the maximum depth or thickness of an SO-DIMM as 4.0 mm. That specification is also a maximum; modules that exceed the specified thickness can cause reliability problems.

Electrical Design of the RAM SO-DIMM

The electrical characteristics of the RAM SO-DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7. To find out how to obtain the specification, see RAM Expansion Modules.

The specification defines several attributes of the DIMM, including storage capacity and configuration, connector pin assignments, and electrical loading. The specification supports SO-DIMMs with either one or two banks of memory.

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 iBook 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 (200-pin SDRAM SO–DIMM, PD INFORMATION) of the JEDEC standard 21-C specification, release 7.

DDR SDRAM Devices

The DDR SDRAM devices used in the RAM expansion modules must be self-refresh type devices for operation from a 2.5 V power supply.

The devices are programmed to operate with a CAS latency of 2.5 or 3. At these CAS latencies, the access time from the clock transition must be 7.5 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.

Configuration of RAM SO-DIMMs

Table 4-1 shows information about the different sizes of SO-DIMMs used in the iBook. The first three columns show the memory size, configuration, and number of banks in the SO-DIMMs. The other three columns show the number, density, and configuration of the DDR SDRAM devices making up the memory modules.

**Table 4-1** Sizes of RAM expansion modules and devices
SO-DIMM size	SO-DIMM configuration (MB x bits)	Number of devices	Device configuration (MB x bits)	Number of banks
128 MB	16 x 64	8	16 x 8	1
128 MB	16 x 64	8	8 x 16	2
256 MB	32 x 64	16	16 x 8	2
256 MB	32 x 64	8	32 x 8	1
256 MB	32 x 64	8	16 x 16	2
512 MB	64 x 64	8	64 x 8	2
512 MB	64 x 64	16	32 x 8	2
1 GB	128 x 64	16	64 x 8	2

Address Multiplexing

Signals A[0] – A[12] and BA[0] – BA[1] on each DDR SDRAM SO-DIMM make up a 15-bit multiplexed address bus that can support several different types of DDR SDRAM devices. Table 4-2 lists the types of devices that can be used in the iBook by size, configuration, and sizes of row, column, and bank addresses.

**Table 4-2** Types of DDR DRAM devices
Device size	Device configuration (bytes x bits x banks)	Row address bits	Column address bits
128 Mbits	4 M x 8 x 4	12	10
128 Mbits	2 M x 16 x 4	12	9
128 Mbits	1 M x 32 x 4	12	8
256 Mbits	8 M x 8 x 4	13	10
256 Mbits	4 M x 16 x 4	13	9
512 Mbits	16 M x 8x 4	13	11

DDR RAM SO-DIMM Electrical Limits

Each RAM SO-DIMM must not exceed the following maximum current limits on the +2.5 V supply:

Active: 1.2 A (maximum of 8 devices per bank, 150 mA per device)

Sleep: 12 mA (total for all banks)

The maximum current specified for active operation generally rules out the use of 4-bit-wide DDR SDRAM devices in a RAM expansion module. Such a module would have 16 such devices per bank, and the 1.2 A maximum current would allow only about 75 mA per device. To stay within the current limits, RAM expansion modules should use only 8-bit or 16-bit DDR SDRAM devices.

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