CY7C1310AV18
CY7C1312AV18
CY7C1314AV18
PRELIMINARY
Document #: 38-05497 Rev. *A
Page 14 of 21
is loaded into the instruction register upon power-up or
whenever the TAP controller is given a test logic reset state.
SAMPLE Z
The SAMPLE Z instruction causes the boundary scan register
to be connected between the TDI and TDO pins when the TAP
controller is in a Shift-DR state. The SAMPLE Z command puts
the output bus into a High-Z state until the next command is
given during the “Update IR” state.
SAMPLE/PRELOAD
SAMPLE / PRELOAD is a 1149.1 mandatory instruction.
When the SAMPLE / PRELOAD instructions are loaded into
the instruction register and the TAP controller is in the Cap-
ture-DR state, a snapshot of data on the inputs and output pins
is captured in the boundary scan register.
The user must be aware that the TAP controller clock can only
operate 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 possi-
ble that during the Capture-DR state, an input or output will
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 will capture the
correct value of a signal, the SRAM signal must be stabilized
long enough to meet the TAP controller's capture set-up plus
hold times (t
CS
and t
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 still possible to capture all other signals
and simply ignore the value of the CK and CK# captured in the
boundary 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 bound-
ary scan register between the TDI and TDO pins.
PRELOAD allows an initial data pattern to be placed at the
latched parallel outputs of the boundary scan register cells pri-
or to the selection of another boundary scan test operation.
The shifting of data for the SAMPLE and PRELOAD phases
can occur concurrently when required - that is, while data
captured is shifted out, the preloaded data can be shifted in.
BYPASS
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 the TDI and TDO pins. The
advantage of the BYPASS instruction is that it shortens the
boundary scan path when multiple devices are connected
together on a board.
EXTEST
The EXTEST instruction enables the preloaded data to be
driven out through the system output pins. This instruction also
selects the boundary scan register to be connected for serial
access between the TDI and TDO in the shift-DR controller
state.
EXTEST OUTPUT BUS TRI-STATE
IEEE Standard 1149.1 mandates that the TAP controller be
able to put the output bus into a tri-state mode.
The boundary scan register has a special bit located at bit #47.
When this scan cell, called the "extest output bus tristate", is
latched into the preload register during the "Update-DR" state
in the TAP controller, it will directly control the state of the
output (Q-bus) pins, when the EXTEST is entered as the
current instruction. When HIGH, it will enable the output
buffers to drive the output bus. When LOW, this bit will place
the output bus into a High-Z condition.
This bit can be set by entering the SAMPLE/PRELOAD or
EXTEST command, and then shifting the desired bit into that
cell, during the "Shift-DR" state. During "Update-DR", the
value loaded into that shift-register cell will latch into the
preload register. When the EXTEST instruction is entered, this
bit will directly control the output Q-bus pins. Note that this bit
is pre-set HIGH to enable the output when the device is
powered-up, and also when the TAP controller is in the
"Test-Logic-Reset" state.
Reserved
These instructions are not implemented but are reserved for
future use. Do not use these instructions.
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