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TAP instruction set
Eight different instructions are possible with the 3-bit instruction register. All combinations are listed in the Instruction Codes table. Three of
these instructions are reserved and should not be used.
Note that the TAP controller used in this SRAM is not fully compliant to the 1149.1 convention because some of the mandatory 1149.1
instructions are not fully implemented. The TAP controller cannot be used to load address, data, or control signals into the SRAM and cannot
preload the I/ O buffers. The SRAM does not implement the 1149.1 commands EXTEST or INTEST or the PRELOAD portion of SAMPLE/
PRELOAD. Instead, it performs a capture of the I/ O ring when these instructions are executed.
Instructions are loaded into the TAP controller during the Shift-IR state when the instruction register is placed between TDI and TDO. During
this state, instructions are shifted through the instruction register through the TDI and TDO pins/ balls. To execute the instruction once it is
shifted in, the TAP controller needs to be moved into the Update-IR state.
EXTEST
The EXTEST instruction, which executes whenever the instruction register is loaded with all 0s, is not implemented in this SRAM TAP
controller. The TAP controller, however, does recognize an all-0 instruction. When an EXTEST instruction is loaded into the instruction register,
the SRAM responds as if a SAMPLE/ PRELOAD instruction has been loaded. Unlike the SAMPLE/ PRELOAD instruction, EXTEST places the SRAM
outputs in a High-Z state.
EXTEST is a mandatory 1149.1 instruction. this device, therefore, is not compliant with 1149.1.
IDCODE
The IDCODE instruction is loaded into the instruction register upon power-up or whenever the TAP controller is given a test logic reset state.
The IDCODE instruction causes a vendor-specific, 32-bit code to be loaded into the instruction register. It also places the instruction register
between the TDI and TDO pins/ balls and allows the IDCODE to be shifted out of the device when the TAP controller enters the Shift-DR state.
SAMPLE Z
The SAMPLE Z instruction causes the boundary scan register to be connected between the TDI and TDO pins/ balls when the TAP controller is
in a Shift-DR state. It also places all SRAM outputs into a High-Z state.
SAMPLE/ PRELOAD
When the SAMPLE/ PRELOAD instruction is loaded into the instruction register and the TAP controller is in the Capture-DR state, a snapshot of
data on the inputs and bidirectional pins/ balls is captured in the boundary scan register. Note that the SAMPLE/ PRELOAD is a 1149.1
mandatory instruction, but the PRELOAD portion of this instruction is not implemented in this device. The TAP controller, therefore, is not
fully 1149.1 compliant.
Be aware that the TAP controller clock can operate only at a frequency up to 10 Mhz, while the SRAM clock operates more than an order of
magnitude faster. Because there is a large difference in the clock frequencies, it is possible that during the Capture-DR state, an input or output
can undergo a transition. The TAP may then try to capture a signal while in transition (metastable state). This will not harm the device, but
there is no guarantee as to the value that will be captured. Repeatable results may not be possible.
To guarantee that the boundary scan register captures the correct value of a signal, the SRAM signal must be stabilized long enough to meet the
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TAP controller’s capture setup plus hold time ( CS plus CH). The SRAM clock input might not be captured correctly if there is no way in a
design to stop (or slow) the clock during a SAMPLE/ PRELOAD instruction. If this is an issue, it is possible to capture all other signals and
ignore the value of the CK and CK# captured in the bounder scan register.
Once the data is captured, it is possible to shift out the data by putting the TAP into the Shift-DR state. This places the boundary scan register
between the TDI and TDO pins.
Note that since the PRELOAD part of the command is not implemented, putting the TAP to the Update-DR state while performing a SAMPLE/
PRELOAD instruction will have the same effect as the Pause-DR command.
BYPASS
The advantage of the BYPASS instruction is that it shortens the boundary scan path when multiple devices are connected together on a board.
When the BYPASS instruction is loaded in the instruction register and the TAP is placed in a Shift-DR state, the bypass register is placed between
TDI and TDO.
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