CY7C1386DV25, CY7C1386FV25
CY7C1387DV25, CY7C1387FV25
Document Number: 38-05548 Rev. *E
Page 12 of 30
Instruction Register
Three-bit instructions can be serially loaded into the instruction
register. This register is loaded when it is placed between the
TDI and TDO balls as shown in the
TAP Controller Block
Diagram
. Upon power up, the instruction register is loaded
with the IDCODE instruction. It is also loaded with the IDCODE
instruction if the controller is placed in a reset state as
described in the previous section.
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 boundary scan register is loaded with the contents of the
RAM input and output 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 input
and output 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 on page 15
.
TAP Instruction Set
Overview
Eight different instructions are possible with the three bit
instruction register. All combinations are listed in
Identification
Codes on page 15
. Three of these instructions are listed as
RESERVED and should not be used. The other five
instructions 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.
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.
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 balls when the TAP
controller is in a Shift-DR state. The SAMPLE Z command
places all SRAM outputs into a High-Z 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 input 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. As 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 setup 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 pri-
or to the selection of another boundary scan test operation.
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