System Protection Features White Paper
The Agilent Medalist i5000 has several features that protect the test system as well as the devices on the board under test. The features described here enhance the reliability and thus the availability of the Medalist i5000 test system by protecting it from damage to the electrical circuits by invalid voltages on the board under test. These features include:
• DriveCheck – verifies that the digital driver can transition to all programmed logic states.
• Overvoltage/undervoltage protection – protects the system
drivers and receivers from excessively high or low voltages on the board under test.
• On-board discharge – hardware on the unmultiplexed hybrid 144
card to discharge trapped charges on the board under test which prevents damage to the pin card relays.
• Current limit – prevents the DUT power supplies from supplying more than a specified level of current. This feature also protects the board under test.
This paper explores these features in detail. It also discusses ways to over come problems on the board under test which may cause these protection schemes to be activated.
During a digital test, the DriveCheck feature on the Medalist i5000 system senses whether a driver reaches its programmed logic level during vector execution. If this condition is not met, a fatal error message will occur during test execution, halting the test. The report message will list a DriveCheck error condition, the node the error occurred on, and the vector number when appropriate. DriveCheck guarantees that the digital driver is outputting the correct state to the device under test by validating the driver state during the receive strobe. In addition, DriveCheck may provide clues into unstable tests if other devices attached to the same node indicted are not appropriately disabled where their outputs could influence the device test. A DriveCheck failure can be caused by interference with other devices on the board under test. Conditions that can cause this failure are:
• A low impedance path between the Agilent Medalist i5000
driver and a high current source on the board. This source would have to be disabled in order to prevent this runtime
error from occurring.
• An oscillator on the board that is not being disabled. The
transitions of this oscillating voltage on the Medalist i5000 driver can cause this error to be triggered. Disabling the oscillator will eliminate this error.
• Device input pins that are not connected. The error mechanism
for unconnected input pins on a device on the board is similar to the oscillator case above. You can prevent this type of error by putting the driver at a default state or by making the pin assignment bi-directional and setting a pull-up or pull-down load on the pin. The error can also be corrected by putting pull-up or pull-down resistors on those pins in the test fixture. Very high current sources on the board that cannot be disabled will require that no driver exists on the pin exhibiting the error. In cases where the there is a DriveCheck error that cannot be resolved using the techniques listed above, the pin should be “starred-out” (i.e., put an “*” for the pin number and make that
pin a “non-digital” pin) and the test must be recompiled. This
will remove the digital driver from the pin for the digital test. Undervoltage / Overvoltage Protection The Overvoltage/Undervoltage protection feature on the Medalist i5000 enhances the reliability of your test system by protecting the test system drivers and receivers from unexpected high or negative voltages on the board under test. If an overvoltage or undervoltage condition occurs, it automatically disconnects the digital drivers and receivers from the board under test and prevents the digital or mixed test from executing.
How overvoltage/undervoltage protection operates Each unmultiplexed Hybrid 144 card in the Medalist i5000 has circuitry to detect an overvoltage or negative voltage (undervoltage) condition which could damage the digital driver or receiver on the card. This circuitry is used during the run of the testplan to see if a potentially damaging condition occurs on any pin of the unmultiplexed Hybrid 144 card. The Overvoltage/undervoltage protection works as follows: 1. When the BT-Basic commands cps,sps,gpconnect, or
gpdisconnect are executed by the testplan or from the
BT-Basic command line, a flag is set in the test system indicating that an OV-UV scan is required.
2. If the testplan or user executes a digital or mixed test and the OV-UV Scan flag is set, then that testhead scans ALL pins on the unmultiplexed Hybrid 144 pin cards. This includes pins not listed in the wirelist file of the board directory. This scan occurs on all unmultiplexed Hybrid 144 pin cards in all modules in the test system. Any digital channel that fails this OV-UV scan will be disabled until reset by another OV-UV scan or by any of the commands listed above. The safe range of voltages for a channel is from –0.25V to +6.0V. Once the OV-UV scan is complete, the OV.UV Scan flag is reset.
3. Pins that fail the initial OV-UV scan will be checked one more time. If they still fail, the pins will be placed on the disabled resource list. This second check is to eliminate the chance that a transient voltage could falsely indict a pin. 4. If any mixed or digital test attempts to use any resource marked as failing the OV-UV scan, the test execution will halt with a fatal error. The test failure message will report an over voltage error condition exists and will also report the BRC, node name and device pin number of the pin that failed the OV-UV scan.
5. When the testplan or user executes an unpowered,load
board,scratch board, or testhead is 1 command
or system diagnostics is run on the Medalist i5000, the disabled channels are restored to full functionality. Note that the OV-UV scan can also be initiated from the BT-Basic command line by using the “verify channels” command. The
verify channels command is available from the Push-Button Debug interface as well. This command is not used in the testplan since there is already a mechanism to automatically run the OV-UV scan.
How to correct for overvoltage/undervoltage errors The only way to correct for an OV/UV error is to remove the source of the error. Look for connections to unsafe voltages on the board under test that are connected via analog components (resistors, jumpers, etc.) to the device that you are having the overvoltage or undervoltage errors on. In some cases, if the pin does not need to be used by the digital test, you can “star-out” the pin such that
no digital resource is assigned to that pin. If you do this, make sure the wirelist file does not have any digital resources assigned to that node. To do this, do the following procedure: 1. Launch a BT-Basic window from the Medalist i5000 software. 2. Execute „list object “wirelist.o” over “wirelist” at the
BT-Basic command line to create an up-to-date wirelist file. 3. Execute a „load “wirelist”‟ at the BT-Basic command line
to load the wirelist file into the BT-Basic workspace. 4. Use the “find” command to search for the device test
name that fails the OV/UV check in the wirelist file (e.g., find “u35”).
5. Verify that there are no digital resources assigned to the unsafe node for that test. If there are, remove them from the file
6. Re-save the wirelist file (re-save).
7. Compile the wirelist file (compile “wirelist”).
8. Re-compile the digital or mixed test that you were having the overvoltage or undervoltage error on (compile
9. Re-execute the test in the debug environment to make sure that no overvoltage or undervoltage errors occur.
The Agilent Medalist i5000 also includes a capacitor discharge solution that provides the following benefits:
• Protects the Medalist i5000 unmultiplexed Hybrid 144 card
relays by eliminating trapped charges on the board. These charges can exist on capacitors that were charged during the testing of the board.
• Eliminates voltage on capacitors that could cause analog
in-circuit test measurements to be erroneous.
The capacitor discharge algorithm on the Medalist i5000 is different than on the Medalist 3070. The Medalist i5000 will execute the “.discharge” test that was created by the program
generator when the testplan executes the “unpowered”
command just like the Medalist 3070. However, the Medalist i5000 also executes an additional discharge algorithm on all pins of the unmultiplexed Hybrid 144 card. This additional discharge is referred to as the hardware discharge.
The new hardware discharge algorithm
This algorithm is described below:
1. The first pin on the unmultiplexed Hybrid 144 card is checked for a voltage greater than 50mV.
2. If the voltage on the pin is less than the 50mV threshold, then the pin is connected to system ground and the discharge check continues with the next pin.
3. If the voltage on the pin is greater than the 50mV threshold, then a discharge circuit is connected to the pin on the unmultiplexed Hybrid 144 card and the pin is checked again. It will be checked for several times until it is below the threshold or the maximum number of attempts to discharge the pin has been exceeded. If it has been discharged, then the pin is connected to system ground and the discharge check continues with the next pin.
4. If the pin never goes below the 50mV threshold, then a failure is generated and the failure message indicts the pin (i.e., BRC) on which the failure occured.
5. This algorithm repeats steps 1 through 4 above until all 144 channels of the unmultiplexed Hybrid 144 card are checked. Each pin is checked even though it may not be connected to the board under test. This is because this is a hardware algorithm embedded on the unmultiplexed Hybrid 144 card that is executed when ever there is a discharge test requested (e.g., unpowered,dps,faoff,
Note that this hardware discharge is executed on all unmultiplexed Hybrid 144 cards in parallel. This speeds the overall execution of the discharge algorithm. The total execution time of the hardware algorithm is less than 500mS (assuming there were no failures). Causes for discharge failures
Situations that can cause a failure with discharge are batteries or “super caps” on the board that are not properly discharged.
These devices should be discharged through GP relays or other fixture electronics. Batteries, generally, should not be installed on the board during test. If batteries must be on the board during test, you should avoid probing the nodes directly connected to the batteries.
Current Limit protects both the board under test and the DUT power supplies from shorts on the board, by limiting the current that can be drawn from the supplies. The maximum current is programmable and is set by the BT-BASIC sps statement in the
testplan. If you are debugging a test and suspect that current limit has occurred, you can use the BT-BASIC pslimit statement
to determine the status of the DUT power supplies. Note that you can also use the rps command to read the actual voltage and
current being output by the DUT supply. Please refer to the online help system for more information on the sps,rps, and pslimit
commands. Note that running the testplan on a board with a current limit problem will cause the board to fail and the failure message will indict the failing DUT power supply.
The Agilent Medalist i5000 provides several mechanisms to protect your investment in the Medalist i5000 test system. It also helps to reduce the risk of potential damage to the board under test and maximizes test system availability. System availability is a significant factor when considering the value of the test system. www.agilent.com Agilent Technologies’ Test and Measurement Support, Services, and Assistance Agilent Technologies aims to maximize the value you receive, while minimizing your risk and problems. We strive to ensure that you get the test and measurement capabilities you paid for and obtain the support you need. Our extensive support resources and services can help you choose the right Agilent products for your applications and apply them successfully. Every instrument and system
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