4.2 Rapid Shutdown

Overview

For the COMe-bSC2 ECC XT (38018-xxxx-xx-x), Kontron has implemented a rapid shutdown function. It works as follows:

1) An active-high shutdown signal is asserted by the COM Express Eval Type 2 carrier board via pin C67 of the COM Express

connector. The characteristics of the shutdown signal are as follows:

» Amplitude 5.0V +/- 5%

» Source impedance < = 50 ohms

» Rise time 1uS⇐

» Duration >= 20uS

The assertion of this signal causes all COMe-bSC2 ECC XT power regulators to be disabled and the internal power supply

rails to be discharged by crowbar circuits. The shutdown circuitry provides internal energy storage that maintains crowbar

activation for at least 2mS following the de-assertion of the shutdown signal. The circuit also incorporates a weak input

Crowbar implementation details

As a tool for designing the internal crowbars, Kontron developed tallied the total capacitance present on each of the

COMe-bSC2 ECC XT internal power rails, and calculates the required discharge resistance in order to achieve the desired

voltage decay time constant. The principal design criteria are that each supply rail must decay to 37% of initial value

(equivalent to 1RC) within 300uS, and to below 1.5V within 2mS. Analysis of the results shows that the 25 or so power

rails in the COMe-bSC2 ECC XT design fall into four general classes. Each class of power rails has a corresponding discharge

strategy.

1) Power Input Rails: The main 12V power input rail incorporates about 300uF of distributed capacitance. This rail must be

discharged by an external crowbar located on the carrier board, which must provide a shunt resistance of approximately 1

ohm. The peak power dissipation in this crowbar resistance will be relatively high (on the order of 150W when the crowbar

single resistor or multiple smaller resistors may be used to achieve sufficient pulse handling capability.

Because of the relatively high currents in the discharge paths, these crowbar circuits require wide copper traces and

careful component placement adjacent to the output components of the corresponding power supplies.

4.18 Shared Graphics Interfaces

On COM Express® pin-out Type 2 based Computer-on-Modules the PCI Express Graphics interface is usually multiplexed

with several digital display interfaces like SDVO, DisplayPort, HDMI or TMDS if supported by the chipset. The new

generation Huron River platform no longer shares it's interfaces. For full backwards compatibility the COMe-bSC2 includes

PEG Interface pin-outs