GB2169087A

GB2169087A - Testing electronic circuit assemblies including crystal oscillators

Info

Publication number: GB2169087A
Application number: GB08530302A
Authority: GB
Inventors: Roger John Livesey; Christopher John Sherratt
Original assignee: Fujitsu Services Ltd
Current assignee: Fujitsu Services Ltd
Priority date: 1984-12-20
Filing date: 1985-12-09
Publication date: 1986-07-02
Also published as: GB8432305D0; GB2169087B; GB8530302D0

Abstract

Testing an electronic circuit assembly 5 incorporating a crystal oscillator, while the oscillator is running, is made possible by overdriving the oscillator to reduce its operational frequency or to inhibit its operation entirely. The oscillator is overdriven by a signal which is applied to the output of the oscillator via a connection socket 13 and a probe 3, the signal being amplified by an amplifier in a circuit 1 mounted in close proximity to the probe. <IMAGE>

Description

SPECIFICATION A method of and apparatus for testing electronic circuit assemblies The present invention relates to a method of testing electronic circuit assemblies and also to apparatus for use with such a method.

Such circuit assemblies are often complex structures comprising interconnected circuit elements mounted on a printed circuit board and are arranged for digital operation. One technique for simplifying the location of faults is known as 'in-circuit' testing. Using this technique test signals are injected at various points into a circuit and outputs observed.

When, however, a clock pulse generator such as, for example, a free running crystal oscillator is included in the circuit, it is sometimes impossible to test other components connected to the output of the oscillator because of the influence of its high frequency output on these components.

It has previously been proposed in this situation to disconnect the oscillator from the other components on the circuit board under test, however, the construction of present day circuit assemblies is such that isolation of a circuit component by disconnection is not feasible.

According to one aspect of the present invention, a method of testing an electronic circuit assembly including a crystal oscillator, comprises the step of applying a signal to the output of the oscillator to thereby overdrive the oscillator and to force its output to a predetermined state.

According to another aspect of the present invention, apparatus for testing an electronic circuit assembly including a crystal oscillator comprises; a test signal generator; a plurality of test probes, at least some of which, in operation, apply test signals from the test signal generator, to the electronic circuit assembly, one of the probes contacting an output of the oscillator; and an amplifier mounted in close physical proximity to the probe contacting the oscillator output and effective to apply an amplified test signal from the test signal generator, to the oscillator output so as to overdrive the oscillator and force its output to a predetermined state.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which; Figure 1 is a schematic diagram of a multifunction test apparatus, and Figure 2 shows, schematically, the apparatus in operation.

Referring to the drawing, the test apparatus 1 is arranged to operate in three modes, an overdrive mode, a frequency monitor mode and a short circuit test mode. It is used in conjunction with a so-called 'bed-of-nails' device 2 (shown in Fig. 2) which comprises a large number of spring loaded contact probes 3 for contact with test points 4 on a printed circuit board 5 under test. Test signals are applied to test points on the printed circuit board 5 through selected ones of the contact probes 3 from in-circuit test apparatus 6.

The components of the test apparatus 1 are mounted on a small printed circuit board which is located on the 'bed-of-nails' device behind the test probes. This arrangement enables short electrical connections to be made between the circuitry of the test apparatus and some of the probes 3 of the bed-of-nails' device 2. This is an important feature of the invention and is necessary to keep the impedance of the connections to a minimum.

The test apparatus 1 comprises an amplifier 7, a pair of relays 8 and 9 and a high impedance buffer amplifier/driver 10. Connection sockets 11, 12 and 13 are provided for electrical connection to probes of the 'bed-ofnails' device 2. The socket 11 is a positive 5 volt input socket, which draws its power from the circuit board under test via one of the probes 3, to supply the amplifier 7, the relay coils 8 and 9 and the high impedance amplifier/driver 10. The socket 12 is a 0 volt input connection which is connected to the 0 volt line of the circuit under test via another probe 3 and the socket 13 carries an output signal from the apparatus 1 which is applied to the crystal oscillator on the circuit board under test via a further contact probe 3.

A wire wrap pin 14 is provided for the input of test signals and wire wrap pins 15 and 16 are for connection to relay driver circuits (not shown) for the relays 8 and 9 respectively. A further wire wrap pin 17 enables the free running frequency of the crystal oscillator to be monitored.

The contacts of the relays 8 and 9 are operative to switch the circuitry of the test apparatus between its three operating modes.

The contacts are shown in their unoperated positions, i.e. when the relay coils are unenergised.

The three modes of operation of the test apparatus will now be described beginning with the overdrive mode. In order to select the overdrive mode, both relays 8 and 9 must be energised. Such energisation causes the movable contacts to change over so that they now contact their other respective fixed contacts. In this mode it will be seen that signals applied to the test signal input pin 14 will be amplified by the amplifier 7 and passed to the output socket 13. The amplified signals, which are applied to the output of the crystal oscillator via the probe of the 'bed-of-nails' device, are effective to overdrive the output of the crystal oscillator. Thus, the operation of the crystal oscillator can be stopped entirely, or its frequency of operation modified, in accordance with the signals applied at the input pin 14.By controlling the crystal oscillator in this manner, it will be realised that comprehensive testing of all other components connected to the output of the crystal oscillator on the circuit board under test is now possible. It has been found in practice that effectively reducing the operating frequency of a high frequency free running oscillator down to 2MHz, for example, enables much of the necessary testing to be accomplished.

In a second mode of operation the test apparatus enables the free running frequency of the crystal oscillator to be monitored without substantially increasing the loading on the oscillator. This mode of operation is selected by energising relay 8 only, to cause its movable contact to move to its other fixed contact. In this mode the output socket 13 and thus the output of the crystal oscillator is disconnected from the test input pin 14 and is connected to the high impedance buffer amplifier/driver 10. The output from the amplifier/ driver 10 is connected to wire wrap pin 17 and thus the frequency of the crystal oscillator may be monitored by attaching a frequency counter to the pin 17.

The third mode of operation enables a test for short circuits to be carried out on the board under test. In this mode no power is applied to the circuit board under test and therefore no power is applied to the test apparatus 1. Neither of the relays 8 and 9 is energised and thus their movable contacts are in the positions shown in Fig. 1. In this case the output socket 13 is connected directly to the test input pin 14 and thus the output of the crystal oscillator is connected to the pin 14. A test for short circuits, using the pin 14 can now be carried out between the output of the crystal oscillator, together with all the other components connected thereto, and other parts of the circuit under test.Although the high impedance amplifier/driver 10 is still connected by the contacts of relay 9 to the crystal oscillator, this does not impair the short cicuit testing as no power is applied to the circuitry at this time.

It will be realised that the prime function of the described arrangement is the crystal oscillator overdrive facility enabling comprehensive testing of other components on the circuit board under test to be carried out. However, the frequency monitor mode of operation together with the shorts test mode provides further useful testing facilities.

It has been found that, in order to obtain effective overdrive of the oscillator, the described arrangement must be mounted within 2 inches of the crystal oscillator. Mounting the printed circuit board carrying the apparatus directly on to the 'bed-of-nails' device as previously described ensures that the necessary close coupling is attained.

The described arrangement has been found effective to overdrive oscillators having a free running frequency of up to 20MHz.

Claims

1. A method of testing an electronic circuit assembly including a crystal oscillator, comprising the step of applying a signal to the output of the oscillator to thereby overdrive the oscillator and to force its output to a predetermined state.

2. A method as claimed in claim 1, in which the signal is effective to reduce the operational frequency of the oscillator.

3. Apparatus for testing an electonic circuit assembly including a crystal oscillator comprising; a test signal generator; a plurality of test probes, at least some of which, in operation, apply test signals from the test signal generator, to the electronic circuit assembly, one of the probes contacting an output of the oscillator; and an amplifier mounted in close physical proximity to the probe contacting the oscillator output and effective to apply an amplified test signal from the test signal generator, to the oscillator output so as to overdrive the oscillator and force its output to a predetermined state.

4. Apparatus as claimed in claim 3, in which the signal is effective to reduce the operational frequency of the oscillator.

5. Apparatus as claimed in claim 3 or 4, including a frequency monitoring device and switching means for connecting the output of the oscillator to either the test signal generator or to the frequency monitoring device.

6. A method of testing an electronic circuit assembly including a crystal oscillator substantially as hereinbefore described with reference to the accompanying drawing.

7. Apparatus for testing an electronic circuit assembly including a crystal oscillator constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawing.