USING PRELOAD AND OBSERVABILITY
Inordertobetestable, acircuitmustbebothcontrollable
and observable. To achieve this, the MACH devices
incorporate register preload and observability.
Preloaded
HIGH
D
Q1
In preload mode, each flip-flop in the MACH device can
be loaded from the I/O pins, in order to perform
functional testing of complex state machines. Register
preload makes it possible to run a series of tests from a
known starting state, or to load illegal states and test for
proper recovery. This ability to control the MACH
device’s internal state can shorten test sequences,
since it is easier to reach the state of interest.
Q
AR
Preloaded
HIGH
The observability function makes it possible to see the
internal state of the buried registers during test by
overriding each register’s output enable and activating
the output buffer. The values stored in output and buried
registers can then be observed on the I/O pins. Without
this feature, a thorough functional test would be
impossible for any designs with buried registers.
D
Q2
Q
AR
While the implementation of the testability features is
fairly straightforward, care must be taken in certain
instances to insure valid testing.
Onecaseinvolvesasynchronousresetandpreset. Ifthe
MACH registers drive asynchronous reset or preset
lines and are preloaded in such a way that reset or
preset are asserted, the reset or preset may remove the
preloaded data. This is illustrated in Figure 8. Care
should be taken when planning functional tests, so that
states that will cause unexpected resets and presets are
not preloaded.
On
Off
Preload
Mode
Q1
Another case to be aware of arises in testing combinato-
rial logic. When an output is configured as combinato-
rial, the observability feature forces the output into
registered mode. When this happens, all product terms
are forced to zero, which eliminates all combinatorial
data. For a straight combinatorial output, the correct
value will be restored after the preload or observe
function, and there will be no problem. If the function
implements a combinatorial latch, however, it relies on
feedback to hold the correct value, as shown in Figure 9.
As this value may change during the preload or observe
operation, you cannot count on the data being correct
after the operation. To insure valid testing in these
cases, outputs that are combinatorial latches should not
be tested immediately following a preload or observe
sequence, but should first be restored to a known state.
AR
Q2
Figure 8. Preload/Reset Conflict
16751E-32
Set
All MACH 2 devices support both preload and
observability.
Contact individual programming vendors in order to
verify programmer support.
Reset
Figure 9. Combinatorial Latch
16751E-33
29
MACH215-12/15/20
LATTICE [ LATTICE SEMICONDUCTOR ]
