Bit synchronization for high-speed serial device testing
View Patent ↗An apparatus for testing electronic devices employs a programmable device to adjust the timing of the strobes such that the strobes sample the bit stream from a device under test at or near the center of the bit position. The strobe time adjustment is performed based on pairs of strobe readings made around a number of different bit positions. The programmable device examines the pairs of strobe reading made around each of the different bit positions to determine whether or not a bit transition has occurred there. The programmable device selects the bit positions around which a bit transition has not occurred as eye candidates, and defines the center of the largest contiguous region of eye candidates as the center of the bit position.
1. A method of testing an electronic device, comprising:
receiving an electronic signal representing a stream of bits from a device under test;
strobing the bits at two bit positions defined with respect to a number of different reference strobe times to generate at least one pair of strobe readings for each reference strobe time;
selecting one of the reference strobe times that generate strobe readings that indicate an absence of a bit transition, as a reference strobe time; and
testing said electronic device utilizing said reference strobe time.
2. The method according to claim 1 , wherein the strobe readings indicate an absence of a bit transition if the strobe readings are the same.
3. The method according to claim 1 , wherein said strobing and selecting are carried out during initialization phase of testing.
4. The method according to claim 1 , wherein T reference strobe times are used and represent a time series, and each reference strobe time is offset from its adjacent reference strobe time by UI divided by T, where UI is the time length of one bit interval.
5. The method according to claim 4 , wherein said selecting includes:
determining the longest contiguous sequence of reference strobe times that generate strobe readings that indicate an absence of a bit transition; and
selecting a reference strobe time that is at or near the middle of the longest contiguous sequence as the reference strobe time to be used during testing.
6. The method according to claim 1 , wherein multiple pairs of strobe readings are generated for each said reference strobe time.
7. The method according to claim 1 , wherein said selecting includes:
identifying reference strobe times that generate strobe readings that are the same each time a pair of strobe readings are generated for said reference strobe times, and selecting one of the identified reference strobe times as a reference strobe time to be used during testing.
8. A method for determining a reference time to be used in strobing bits in a bit stream generated by an electronic device, comprising:
strobing the bits at two bit positions defined with respect to a first reference time until a predefined number of pairs of strobe readings for said reference time has been strobed;
shifting the reference time by a fixed increment;
strobing the bits at two bit positions defined with respect to the shifted reference time until the predefined number of pairs of strobe readings for said shifted reference time has been strobed;
selecting one of the first reference time or one of the shifted reference times as a reference time to be used in strobing the bits in the bit stream; and
determining functionality of said electronic device utilizing said reference time to analyze said bit stream.
9. The method according to claim 8 , wherein said selecting includes:
identifying a set of reference time candidates, wherein a reference time is identified as a reference time candidate if each of the pairs of strobe readings for said reference time reference time outputs the same strobe reading; and
selecting the reference time to be used in strobing the bits in the bit stream from the set of reference time candidates.
10. The method according to claim 9 , wherein said selecting further includes:
determining the longest sequence of reference time candidates; and selecting the reference time candidate that is at or near the center of the longest sequence to be the reference time to be used in strobing the bits in the bit stream.
11. The method according to claim 8 , wherein the two bit positions of the first reference time include a first bit position that is strobed one fixed increment prior to the first reference time and a second bit position that is strobed one fixed increment after the first reference time.
12. The method according to claim 8 , wherein the two bit positions of the shifted reference time include a first bit position that is strobed one fixed increment prior to the shifted reference time and a second bit position that is strobed one fixed increment after the shifted reference time.
13. An apparatus for testing electronic devices, comprising:
a drive circuit for generating signals to be supplied to a device under test in accordance with a test program;
a response circuit for receiving a stream of bits from the device under test and strobing the bits with respect to a reference time to generate strobe readings; and
a device programmed to generate a number of different reference times to be used in strobing the bits, to collect multiple pairs of strobe readings for each of the different reference times, and to select one of the different reference times that generate strobe readings that indicate an absence of a bit transition as the reference time to be used in subsequent strobing.
14. The apparatus according to claim 13 , wherein the different reference times define a sequence of reference times having a common time interval, and the product of the number of different reference times and the common time interval is equal to the time length of one bit interval.
15. The apparatus according to claim 13 , wherein the device is programmed to select a reference time to be used in subsequent strobing from a group of reference time candidates, wherein a reference time is identified as a reference time candidate if each of the pairs of strobe readings for said reference time outputs the same strobe reading.
16. The apparatus according to claim 15 , wherein the device is programmed to determined the longest sequence of reference time candidates and to select the reference time candidate that is at or near the center of the longest sequence to be the reference time to be used in strobing the bits in the bit stream.
17. The apparatus according to claim 13 , wherein the strobe times for each pair of strobe readings are separated from each other by two time increments, wherein one time increment is equal to the time length of one bit interval divided by the number of different reference times.
18. The apparatus according to claim 13 , wherein the device is programmed with a strobe sequence applicable for one test period, wherein the bits are strobed during said one test period based on the strobe sequence and one of the different reference times.
19. The apparatus according to claim 18 , wherein the strobe sequence is defined relative to the beginning of the test period.