Integrated Silicon Solution, Inc. 21
Rev. G
12/02/09
IS61/64LF25636A IS61LF51218A IS61VF25636A IS61VF51218A
TAP INSTRUCTION SET
Eightinstructionsarepossiblewiththethree-bitinstruction
register and all combinations are listed in the Instruction
Code table. Three instructions are listed as
RESERVED
and should not be used and the other five instructions are
describedbelow.TheTAPcontrollerusedinthisSRAM
isnotfullycompliantwiththe1149.1conventionbecause
some mandatory instructions are not fully implemented.
TheTAPcontrollercannotbeusedtoloadaddress,dataor
control signals and cannot preload the
Input
or
Output
buf-
fers. The
SRAM
does not implement the
1149.1
commands
EXTEST
or
INTEST
or the
PRELOAD
portion of
SAMPLE/
PRELOAD
;insteaditperformsacaptureofthe
Inputs and
Output
ring when these instructions are executed. Instruc-
tionsareloadedintotheTAPcontrollerduringtheShift-IR
statewhentheinstructionregisterisplacedbetweenTDI
andTDO.Duringthisstate,instructionsareshiftedfrom
theinstructionregisterthroughtheTDIandTDOpins.To
executeaninstructiononceitisshiftedin,theTAPcontrol-
lermustbemovedintotheUpdate-IRstate.
EXTEST
EXTESTisamandatory1149.1instructionwhichistobe
executed whenever the instruction register is loaded with
all0s.BecauseEXTESTisnotimplementedintheTAP
controller,thisdeviceisnot1149.1standardcompliant.
TheTAPcontrollerrecognizesanall-0instruction.Whenan
EXTESTinstructionisloadedintotheinstructionregister,
theSRAMrespondsasifaSAMPLE/PRELOADinstruction
has been loaded. There is a difference between the instruc-
tions, unlike the
SAMPLE/PRELOAD
instruction,EXTEST
placestheSRAMoutputsinaHigh-Zstate.
IDCODE
The IDCODE instruction causes a vendor-specic, 32-
bit code to be loaded into the instruction register. It also
placestheinstructionregisterbetweentheTDIandTDO
pinsandallowstheIDCODEtobeshiftedoutofthedevice
when theTAP controller enters the Shift-DR state.The
IDCODEinstructionisloadedintotheinstructionregister
uponpower-uporwhenevertheTAPcontrollerisgivena
test logic reset state.
SAMPLE-Z
The SAMPLE-Z instruction causes the boundary scan
registertobeconnectedbetweentheTDIandTDOpins
whentheTAPcontrollerisinaShift-DRstate.Italsoplaces
allSRAMoutputsintoaHigh-Zstate.
SAMPLE/PRELOAD
SAMPLE/PRELOADisa1149.1mandatoryinstruction.The
PRELOADportionofthisinstructionisnotimplemented,so
theTAPcontrollerisnotfully1149.1compliant.Whenthe
SAMPLE/PRELOADinstructionisloadedtotheinstruc-
tionregisterandtheTAPcontrollerisintheCapture-DR
state, a snapshot of data on the inputs and output pins is
captured in the boundary scan register.
ItisimportanttorealizethattheTAPcontrollerclockoper-
atesatafrequencyupto10MHz,whiletheSRAMclock
runsmorethananorderofmagnitudefaster.Becauseof
theclockfrequencydifferences,itispossiblethatduring
theCapture-DRstate,aninputoroutputwillunder-goa
transition.TheTAPmayattemptasignalcapturewhilein
transition (metastable state). The device will not be harmed,
but there is no guarantee of the value that will be captured
or repeatable results.
To guarantee that the boundary scan register will capture
thecorrectsignalvalue,theSRAMsignalmustbestabilized
longenoughtomeettheTAPcontroller’scaptureset-up
plus hold times (tc s and tc h ).ToinsurethattheSRAMclock
input is captured correctly, designs need a way to stop (or
slow)theclockduringaSAMPLE/PRELOADinstruction.
If this is not an issue, it is possible to capture all other
signals and simply ignore the value of the CLK and CLK
captured in the boundary scan register.
Oncethedataiscaptured,itispossibletoshiftoutthedata
byputtingtheTAPintotheShift-DRstate.Thisplacesthe
boundaryscanregisterbetweentheTDIandTDOpins.
Note that since the
PRELOAD
part of the command is not
implemented, putting the
TAP
into the
Update
to the
Update-
DR
state while performing a
SAMPLE/PRELOAD
instruction
willhavethesameeffectasthePause-DRcommand.
BYPASS
When the BYPASS instruction is loaded in the instruc-
tionregister and theTAP isplaced in aShift-DR state,
thebypassregisterisplacedbetweentheTDIandTDO
pins.TheadvantageoftheBYPASSinstructionisthatit
shortens the boundary scan path when multiple devices
are connected together on a board.
RESERVED
These instructions are not implemented but are reserved
forfutureuse.Donotusetheseinstructions.