CSE1CT Computer Technology

Motherboards and Chipsets

Motherboards

The "motherboard" is the main printed circuit board in the computer.

In early PC designs, the motherboard carried the CPU, memory and "glue" chips. Everything else was mounted on printed circuit boards that plugged into the motherboard.

The trend now is to put everything on the motherboard. This has some positive points and some negatives.

• The manufacturer can create a system that works.
• Reduces cost.
• Some integrated functions can be replaced by expansion cards.

• If there is no AGP socket, you are stuck whith the on board graphics.
• Any failure is catastrophic - usually the whole board has to be replaced.
• The manufacturer may skimp on performance (eg graphics controller may use main memory).

• Socket for CPU module
• Socket(s) for memory
• Cache memory (if not part of CPU module)
• Expansion sockets (PCI, AGP and maybe ISA)
• Interface chips (the chipset)
• Video controller
• Disc interface (hard & floppy)
• Keyboard interface
• BIOS & CMOS memories
• Real Time clock

Motherboard Form Factor

The "form factor" of a motherboard defines:

• Overall dimensions
• Approximate location of expansion slots
• Approximate location of CPU
• Maximum height of components (varies with position)
• Location of connectors
• Mounting hole locations
• Power connectors

ATX, microATX and FlexATX

The ATX board is derived from the AT (286) motherboard.

The microATX is a shortened version of the ATX.

The FlexATX is slightly smaller than the microATX.

• The ATX may have up to 7 expansion sockets
  including ISA, PCI AGP and PCI/CNR
• The microATX can have up to 4 expansion sockets.
• FlexATX - maximum of 3 expansion sockets.

All versions have a connector "panel" on the rear of the board. There is no standard connector arrangement. The case must provide a standard cutout (hole) of approximately 160 millimetres wide by 44 millimetres wide to accomodate the connectors.

The next diagram shows a typical connector arrangement.

BTX

Announced by Intel in 2003 as the successor to ATX.

It was expected that the BTX form factor will disappearance of PS/2, serial and parallel I/O connectors and that PCI express will be the preferred expansion connector format

ATX is currently (2008) still the most popular form factor.

BTX is designed to support Intel's power hungry CPUs ie. more attention has been paid to cooling requirements.
Basically, the CPU and graphics chips have been physically aligned so that cooling air can be directed over them. Space has been allowed for generous heatsinking of the hot chips (cooling radiators).

The BTX I/O connector arrangement is incompatible with the ATX layout (need a "BTX" case).

The BTX standard allows three different board sizes:

1. picoBTX 8.0" x 10.5" for low profile cases.
2. microBTX up to 10.4" x 10.5" for standard cases.
3. "BTX" up to 12.8" x 10.5" for tower cases.

LPX

NLX

Proprietary Designs

Chipsets

All this logic was constructed from fairly simple logic circuits and required a large motherboard. In addition, it was fairly slow and consumed a relatively large amount of power.

Manufacturers quickly realised that most, if not all, of the functions performed by these individual chips could be integrated into a single Application Specific Integrated Circuit or ASIC. This meant that 50 chips could be replaced by one or two doing the same task. This reduced construction costs, improved reliability, reduced power requirements and reduced mounting area. Performance gains were also made due to the shorted signal paths between the logic circuits, thus allowing them to operate at higher speeds.

• The Northbridge which connects the CPU, memory, PCI bus, AGP graphics adapter and maybe Level 2 cache together.
• The Southbridge which connects the PCI bus to the IDE disc drives (see later lecture), USB bus (later too), ISA bus (if present) and maybe a PC card bus.

The 2006 chipsets for the Pentium family, the "975X" continue this theme (North/South).

• The 82975X Memory Controller Hub connects the CPU to the memory and to the graphics hardware via PCI express connectors.
• The I/O Controller Hub (generation)7 (ICH7) connects to a multitude of I/O functions (eg. Disc, USB, Audio, networking)

The connection between the CPU and the "975" can perform up to 1066 Million transfers per second, each appearing to convey 64 bits. This gives a peak throughput of 8.5GByte/sec.

The chips are joined by the Direct Media Interface (DMI) Bus, a curious design using 4 differential high speed circuits in each direction. This bus carries priority based messages at up to 10Gbit/sec (about 1GByte/sec) each way.

Chipset Ancient History

The first chipset was created in 1986 by a company called "Chips and Technologies." Known as the CS8220, this chipset effectively integrated the motherboard of IBM AT compatible system onto five ICs, the "82C206" and four additional chips that acted as buffers and memory controllers. Later on the four supplementary chips were condensed into three chips creating the New Enhanced AT (NEAT) CS8221 chipset. This was soon followed by the 82C836 Single Chip AT (SCAT) chipset, finally providing an entire chipset in a single IC.

Chipsets, CPUs and New Technologies

Intel Chipsets

Intel has a very large marketshare in the chipset market, being ideally positioned to manufacture chipsets that complement their new processors. Largely due to their success in the chipset market, Intel now sell entire motherboards, utilising their chipsets and processors.

Intel i845 (Pentium 4)

Original Pentium 4 chipsets only provided support for expensive RAMBUS memory. Due to the increasing demand Intel released the i845 chipset, codenamed "Brookdale", allowing the Pentium 4 to be used with PC133 SDRAM. Due to contractual issues, the i845 was re-released early in 2002 with support for the faster DDR SDRAM.

Intel 850 (Pentium 4)

Presumably the first chipset to support the Pentium 4, the Intel 850 chipset provided support for a large range of new features provided by this processor. In particular a "quad-pumped" system bus and support for the expensive and high speed RDRAM.

Intel 820 (Pentium II/III)

Very similar to the Intel 815 chipset, the Intel 820 provides additional features allowing for the use of RDRAM and multiprocessing. Support for UDMA/100 was also introduced.

Intel 815 (Pentium III and Celeron)

Arguably, one of the most commonly used chipsets, the Intel 815 has support for 133MHz FSB, AGP 4X and PC133 SDRAM, features not available on early Pentium III/Celeron chipsets. This chipset does not provide support for multiple processors.

Non-Intel Chipsets

Acer Laboratories Inc (ALi)

ALi manufactures the ALADDiN-P4 chipset, utilising their M1671 North Bridge chip to support the Pentium 4 processor and 400MHz Front Side Bus. Features supported include PC100/PC133 SDRAM or DDR 200/266/333 SDRAM, UDMA 66/100/133 and AGP 4X. This chipset allows for up to 3GB of memory.

VIA

VIA's Apollo P4X266 chipset was responsible for pushing Intel to release the i845 with support for SDRAM. The P4X266 supports the Pentium 4 processor, 400MHz Front Side Bus, DDR 200/266 SDRAM or PC100/PC133 SDRAM, AGP 4X, UDMA/100 and up to 4GB of memory. In addition the South Bridge has integrated network, audio and modem.

References