Part V : Mezzanine Connectors and Power Supply & Environmental Specifications
Mezzanine Connectors
The mezzanine connectors that are specified in the COM Express standard are capable of 6.25GHz operation in order to support PCI Express speeds up to 5GHz as is required for PCI Express Generation 2. These high speed connectors provide sufficient insertion strength to withstand very high shock and vibration acceleration forces that can run into the tens of Gs. There are two connectors required to support the total of 440 pins as specified in the standard. Each connector has two rows of 110 pins each. To form a single 400 pin connector, two individual 220-pin connectors are assembled together in a harness, which makes final assembly on the carrier board easier.
A single 220-pin module-side receptacle is shown in Figure 1. Figure 2 shows a single 220-pin carrier board-side plug. The carrier board plugs come in two heights, allowing for a combined height of receptacle and plug of 5mm and 8mm.
Figure 1. Single 220-pin module-side receptacle
Figure 2. Single 220-pin carrier board-side plug
One source for these connectors is Tyco, which lists the following part numbers for the module-side receptacles:
- 3-1318490-9 or 8-1318490-9 for a single 220-pin receptacle.
- 3-1827231-6 for a single 440-pin receptacle.
For the carrier board-side plugs the company lists the following part numbers:
- 3-1827253-6 for a single 5mm stack height 220-pin plug.
- 3-1827233-6 for a combined 5mm stack height 440-pin plug.
- 3-1318491-6 or 8-1318491-6 for a single 8mm stack height 220-pin plug.
- 3-5353652-6 for a combined 8mm stack height 440-pin plug.
Power Supply Specifications
The COM Express standard specifies a single power rail of 12V as the main power supply for the modules. Next to this power rail, there are two optional ones: (1) a 5V standby power rail for when the main power rail is cut off during power savings mode, and (2) a 3V real-time clock backup power rail for when the module is completely powered down. This power rail is usually connected to a coin cell battery for that matter, whereas the 12V main power rail and the 5V standby power rails are supplied from the system's battery or AC/DC power supply.
This power rail scheme drastically simplifies carrier board design and isolates chipset and module supplier specific design choices for power delivery to the CPU and chipset. The choice of 12V for the main power rail reduces the number of power supply pins needed on the connector and current densities in the power supply. The 12V level also makes it easy to connect to modern Lithium Ion battery packs as used in laptop PCs, which are mostly specified with a 14.4V output level.
Table 1 presents an overview of the power supply specification for pin-out Type 1 and Table 2 for pin-out Type 2-5.
| Power Rail | Voltage | Max Load (W) |
|---|---|---|
| VCC_12V | 12V | 101 |
| VCC_5V_SBY | 5V | 9 |
| VCC_RTC | 3V | . |
Table 1. Power supply specification for pin-out Type 1
| Power Rail | Voltage | Max Load (W) |
|---|---|---|
| VCC_12V | 12 | 160 |
| VCC_5V_SBY | 5 | 9 |
| VCC_RTC | 3 | . |
Table 2. Power supply specification for pin-out Type 2-5
Operating Conditions
The COM Express standard contains environmental specifications for operating temperatures and humidity. It does not contain specifications for shock and vibration. However, placement of module-to-carrier-to-chassis assembly points and mezzanine connector specifications ensure that module manufacturers have room to create excellent shock and vibration performance to withstand acceleration forces that can run into the tens of Gs.
Maximum temperature specifications have been laid out carefully in the standard. This allows any COM Express module's thermal performance to be measured and compared with any other COM Express module. A boon for OEMs who need to be able to swap modules in a single product, without violating system thermal specifications.
Two main thermal values are of special importance here: (a) Tcase_max, and (b) TDPmax. Tcase_max is the maximum temperature the module's heat spreader reaches under TDPmax conditions. TDPmax is the maximum power dissipation of the module, which guarantees that it still operates within the manufacturer's warranty. The COM Express standard mandates that module manufacturers specify the hottest point on the module's heat spreader for Tcase_max measurement purposes. Figure 3 illustrates this.
Figure 3. Tcase_max measurement
End of Part V
