microATX Motherboard Interface Specification Version 1.
microATX Motherboard Interface Specification Version 1.2 IMPORTANT INFORMATION AND DISCLAIMERS 1. INTEL CORPORATION MAKES NO WARRANTIES WITH REGARD TO THIS SPECIFICATION (“SPECIFICATION”), AND IN PARTICULAR DOES NOT WARRANT OR REPRESENT THAT THIS SPECIFICATION OR ANY PRODUCTS MADE IN CONFORMANCE WITH IT WILL WORK IN THE INTENDED MANNER.
microATX Motherboard Interface Specification Version 1.2 Revision History Changes from Version 1.1 to Version 1.2 Global — Replaced ATX 2.01 with ATX 2.1, where appropriate and removed SFX in SFX/SFX12V Power Supply Design Guide. Important Information and Disclaimers updated. Section 2.3.7 and Section3.1 Main Power Connector changed from 20 pin to 24 pin (2 x 12) to support PCIExpress* requirements. Changes from Version 1.0 to Version 1.1 Global—Removed references to AT, Baby AT, and Mini-ATX.
microATX Motherboard Interface Specification Version 1.2 Contents 1. 1.1 1.2 1.3 1.4 1.5 Executive Summary.................................................................................. 6 Other Technical Documents ..................................................................................................6 microATX Form-factor Overview ...........................................................................................7 Benefits to Users ....................................................
microATX Motherboard Interface Specification Version 1.2 6: 7: 8: 9: 10: 11: Example Rear Panel I/O Connector Layout...........................................................................16 microATX Motherboard Maximum Component Height Restrictions ......................................18 20-pin Main Power Supply Connector Configuration (required for microATX) .........................................................................................................19 2x2 Connector Configuration .......
microATX Motherboard Interface Specification Version 1.2 1. Executive Summary microATX is a motherboard form factor developed as a natural evolution of the ATX form factor to address market trends and PC technologies. This specification defines the interface between the motherboard and the chassis.
microATX Motherboard Interface Specification Version 1.2 1.2 microATX Form-factor Overview Figure 1 shows an example of a system using a microATX motherboard. Not to scale Slots for system memory 9.6” (244 mm) Processor location Power supply location 5.25” drive bay for CD-ROM System Fan 3.5” drive bay for FDD ATX-compatible double-high expandable I/O window 6.25” (158.75 mm) x 1.75” (44.
microATX Motherboard Interface Specification Version 1.2 1.3 Benefits to Users While offering the same benefits of the ATX form factor specification, the microATX form factor improves upon the previous specification in several key areas. Current trends in the industry indicate that users require a lower-cost solution for their PC needs. Without sacrificing the benefits of ATX, this form factor addresses the cost requirement by reducing the size of the motherboard.
microATX Motherboard Interface Specification Version 1.2 2. Layout This section describes the mechanical specification of the microATX form-factor motherboard, including physical size, mounting hole placement, connector placement, and component height constraints. 2.1 Board Dimensions Table 3 compares the microATX and ATX 2.11 board dimensions. Table 3: microATX and ATX Board Dimensions Dimension microATX board ATX board, full-sized Maximum width allowable 9.
microATX Motherboard Interface Specification Version 1.2 To avoid damage to traces on microATX and ATX motherboards, chassis standoffs in any locations not specified for microATX and ATX should be removable or not be implemented at all. NOT TO SCALE REAR of board 12” (305 mm) 6.25” wide rear I/O shield 9.6” (244 mm) A B C F PCI/ISA/AGP connector locations (4 max) 9.
microATX Motherboard Interface Specification Version 1.2 2.3 Connector Placement Table 5 lists connector locations. Figure 3 clearly defines the location of the PCI, ISA, and AGP connectors as well as the allowable placement area for I/O connectors on the back panel. The specification provides recommendations, but the exact location of other connectors is left to the judgment of the motherboard designer working in conjunction with the system integrator.
microATX Motherboard Interface Specification Version 1.2 Figure 3: Example microATX Layout Diagram Notes: Datum B 0,0 = mounting location hole B. In this figure, the board is shown oriented with the rear of the board toward the top. See microATX Motherboard Design Suggestions for different combinations of expansion slots.
microATX Motherboard Interface Specification Version 1.2 2.3.2 Disk I/O The exact locations of the floppy, IDE, and/or SCSI I/O connectors are not specified. It is recommended that they be placed along the front edge of the board (oriented as in Figure 3) to the right of the expansion slots. When placing connectors, the designer should keep in mind that proper clearance must be provided for the chassis peripheral bays. 2.3.
microATX Motherboard Interface Specification Version 1.2 Figure 4: Chassis I/O Aperture Requirements Notes: • • • • • • Page 14 Datum B 0,0 = mounting location hole B. Nominal cutout size = 6.25 inches (158.75mm) by 1.75 inches (44.45mm). Distance from top of a typical 0.062 inches (1.57 mm) motherboard to bottom of I/O cutout hole = 0.150 inches (3.81mm). Allowable thickness of a chassis back panel that the I/O shield can clip into is in the range 0.037 inches (0.94mm) to 0.052 inches (1.32mm).
microATX Motherboard Interface Specification Version 1.2 Figure 5 specifies the I/O connector zone. Compliance with this recommendation is necessary to ensure enough clearance between the chassis aperture and motherboard connectors for the I/O shield structure. If the shield provided with the motherboard requires less than the recommended clearance, then the dimensions of the I/O connector area can be waived (hatched area in Figure 5: 5.990 inches [152.15mm] by 1.470 inches [37.34mm]).
microATX Motherboard Interface Specification Version 1.2 Figure 6 shows an example of rear panel I/O connector layout, featuring stacked keyboard and mouse, stacked USB ports, stacked serial and parallel and VGA ports, a LAN port, and stacked audio jacks. This layout is only an example—the microATX form factor allows complete flexibility in the layout of rear panel I/O. For more I/O panel examples, see the microATX System Design Suggestions; for the Web site URL, see Section 1.1.
microATX Motherboard Interface Specification Version 1.2 2.4 Height Constraints 2.4.1 Primary (Component) Side Height Constraints One of the major advantages of the microATX form factor is its backward-compatibility with the ATX specification. The microATX motherboard can be installed in any ATX chassis with the addition of motherboard mounts. Table 6 lists the status of height constraints for specific areas.
microATX Motherboard Interface Specification Version 1.2 (9.600) [243.84] 1.350 [34.29] .400 REF [10.16] (BOARD MTG HOLE) REF (BOARD MTG HOLE) AREA A AREA B 6.100 [154.94] 7.100 [180.34] (9.600) [243.84] 9.200 [233.68] AREA C 2.050 [52.07] 3.750 [95.25] 8.250 [209.55] Area Maximum component height (in inches) A Motherboard component height, 2.80 inches [71.12mm] maximum Chassis clearance over motherboard, 3.0 inches [76.20mm] required Chassis clearance over motherboard, 3.5 inches [88.
microATX Motherboard Interface Specification Version 1.2 3. Power Supply Connector Information 3.1 Main Power Connector The microATX and ATX specifications recommend a 24-pin main connector interface to the power supply (Figure 8). This interface incorporates standard ±5V, ±12V, 3.3V, 5V standby, and soft-power signals. This board-mounted header can be implemented with a Molex* 44206-0007 or equivalent. This mates with the power supply connector, Molex 39-01-2240 or equivalent.
microATX Motherboard Interface Specification Version 1.2 Ground 1 2 Ground +12 V 3 4 +12 V Figure 9: 2x2 Connector Configuration 3.2 Signal Control Definitions 3.2.1 PS-ON PS-ON is an active low TTL signal that turns on all of the main power rails including 3.3V, 5V, -5V, 12V, and -12V power rails. When this signal is held high by the PC board or left open-circuited, outputs of the power rails should not deliver current. These outputs should be held at a zero potential with respect to ground.
microATX Motherboard Interface Specification Version 1.2 3.3 Voltage Tolerances Tolerance for the motherboard power rails should comply to the values listed in Table 7. Table 7: Voltage Tolerances Parameter Range Min. Nom. Max. Unit +12 VDC ±5% +11.40 +12.00 +12.60 Volts + 5 VDC ±5% +4.75 +5.00 +5.25 Volts +3.3 VDC ±5% +3.14 +3.30 +3.47 Volts -12 VDC ± 10 % -10.80 -12.00 -13.20 Volts + 5 VSB ±5% +4.75 +5.00 +5.25 Volts 3.
microATX Motherboard Interface Specification Version 1.2 3.4.1 FanM Signal The FanM signal is an open collector, 2 pulse per revolution tachometer signal from the power supply fan. The signal stops cycling during a lock rotor state; the level can be either high or low. This signal allows the system to monitor the power supply for fan speed or failures. Implementation of this signal would allow a system designer to gracefully power down the system in the case of a critical fan failure.
microATX Motherboard Interface Specification Version 1.2 3.4.3 3.3V Sense Line A remote 3.3 V sense line can be added to the optional connector to allow for accurate control of the 3.3VDC line directly at motherboard loads. Because of potential voltage drops across the connector and traces leading to the motherboard components, it can be advantageous to implement a 3.3V sense line that remotely monitors the 3.3VDC power level at the load on the motherboard.
