CY7C1460AV25
CY7C1462AV25
CY7C1464AV25
Document #: 38-05354 Rev. *D
Page 11 of 27
When the TAP controller is in the Capture-IR state, the two
least significant bits are loaded with a binary “01” pattern to
allow for fault isolation of the board-level serial test data path.
Bypass Register
To save time when serially shifting data through registers, it is
sometimes advantageous to skip certain chips. The bypass
register is a single-bit register that can be placed between the
TDI and TDO balls. This allows data to be shifted through the
SRAM with minimal delay. The bypass register is set LOW
(V
SS
) when the BYPASS instruction is executed.
Boundary Scan Register
The boundary scan register is connected to all the input and
bidirectional balls on the SRAM. The length of the Boundary
Scan Register for the SRAM in different packages is listed in
the Scan Register Sizes table.
The boundary scan register is loaded with the contents of the
RAM I/O ring when the TAP controller is in the Capture-DR
state and is then placed between the TDI and TDO balls when
the controller is moved to the Shift-DR state. The EXTEST,
SAMPLE/PRELOAD and SAMPLE Z instructions can be used
to capture the contents of the I/O ring.
The Boundary Scan Order tables show the order in which the
bits are connected. Each bit corresponds to one of the bumps
on the SRAM package. The MSB of the register is connected
to TDI, and the LSB is connected to TDO.
Identification (ID) Register
The ID register is loaded with a vendor-specific, 32-bit code
during the Capture-DR state when the IDCODE command is
loaded in the instruction register. The IDCODE is hardwired
into the SRAM and can be shifted out when the TAP controller
is in the Shift-DR state. The ID register has a vendor code and
other information described in the Identification Register
Definitions table.
TAP Instruction Set
Overview
Eight different instructions are possible with the three bit
instruction register. All combinations are listed in the
Instruction Codes table. Three of these instructions are listed
as RESERVED and should not be used. The other five instruc-
tions are described in detail below.
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 balls.
To execute the instruction once it is shifted in, the TAP
controller needs to be moved into the Update-IR state.
IDCODE
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 balls and allows
the IDCODE to be shifted out of the device when the TAP
controller enters the Shift-DR state.
The IDCODE instruction 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 Capture-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 20 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
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
boundary 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
prior 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 #89
(for 165-FBGA package) or bit #138 (for 209 FBGA package).
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