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Blinking the LightPath Fault LEDs to extend and enhance observation time Disclosure Number: IPCOM000126964D
Original Publication Date: 2005-Aug-16
Included in the Prior Art Database: 2005-Aug-16
Document File: 3 page(s) / 549K

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One of the service features for the xSeries computers is the LightPath diagnostics. This feature allows a service technician to light a set of diagnostic LEDs, indicating the area of trouble when the system is unpowered. On systems where you cannot remove the covers when the machine is powered, a capacitor is used to provide the necessary energy to light the LEDs for the required minimum time. This capacitor is charged whenever the system is powered. When power is removed from the system, the capacitor must maintain its charge for several minutes and continue to power the I2C devices that hold the LED status. The depression of a switch would connect the capacitor to the LightPath circuit and power the appropriate LEDs, immediately drawing several milliamps of current from the capacitor. The capacitor is only able to supply these milliamps of current for a short period of time. The more LEDs that are illuminated, the greater the current draw on the capacitor. As a result, a very large capacitor must be used in order to provide several seconds of viewing time for the LEDs. This large capacitor requires a large amount of board space. A method of controlling the lightpath diagnostic LEDs, which reduces the size of the capacitor required while still providing the same level of functionility is described. This method is a more informative and more compact solution that saves both precious board space and cost.

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Blinking the LightPath Fault LEDs to extend and enhance observation time

     The current solution for LightPath Diagnostics uses I2C Registers to hold the state of the various error LEDs (Please see Figure 1 below for more details). The system service processor can access these devices over the I2C interface and set the value of each error condition to on or off. The I2C register will hold this value until either the service processor changes or if the chip encounters a power-on reset. Each register can hold the state of eight error LEDs. Additional I2C registers must be added to support greater than eight LEDs.


SPST Switch


 I2C - I/O Expanders 8-Bits Each


470mF - 1F

To Service Processor

8 LED Control Lines Per I/O




Current Limiting Resistor (1 Per LED)

         Figure 1: Example Circuit of Existing Solution When the LightPath button is depressed, all of the enabled error LEDs will be illuminated simultaneously and will continue to remain illuminated until either the capacitor fully discharges or the button is released.

The disadvantages of this existing approach are as follows:

Large Footprint: Space consumed by the I2C devices and the physical size of the large capacitor.

Limited hold time/shelf life: Data held in I2C Registers is lost when the capacitor power is lost .

Limited viewing time: The capacitor quickly begins to discharge once the LEDs are lit. The greater the number of existing error conditions, the faster the capacitor will discharge.

Constantly powering the I2C Registers that hold the LED Data: This means that the Capacitor begins to slowly discharge one its charge current is


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     The solution disclosed in this document uses a microcontroller containing an internal EEPROM to store the faults and control the LED blink rates, thus increasing LED evaluation time while reducing the size of the capactior required. The microcontroller will be powered when the machine is powered-up. When main system power is removed from the machine the microcontroller will also power down. However, the fault content is stored in its nonvolatile EEPROM. The activation of a momentary switch would connect the microcontroller to the lightpath circuit capacitor for power. When the microcontroller is powered it will blink the appropriate LED(s) under program control. Please see Figure 2 below for details

The advantages of this approach are as follows:

Smaller fo...