CY7C1380C
CY7C1382C
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.
To execute the instruction once it is shifted in, the TAP
controller needs to be moved into the Update-IR state.
EXTEST
EXTEST is a mandatory 1149.1 instruction which is to be
executed whenever the instruction register is loaded with all
0s. EXTEST is not implemented in this SRAM TAP controller,
and therefore this device is not compliant to 1149.1. The TAP
controller does recognize an all-0 instruction.
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
When an EXTEST instruction is loaded into the instruction
register, the SRAM responds as if a SAMPLE/PRELOAD
instruction has been loaded. There is one difference between
the two instructions. Unlike the SAMPLE/PRELOAD
instruction, EXTEST places the SRAM outputs in a High-Z
state.
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
(VSS) when the BYPASS instruction is executed.
IDCODE
Boundary Scan Register
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 boundary scan register is connected to all the input and
bidirectional balls on the SRAM. The SRAM has a 75-bit-long
register.
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 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. It also places all SRAM outputs
into a High-Z state.
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.
SAMPLE/PRELOAD
Identification (ID) Register
SAMPLE/PRELOAD is a 1149.1 mandatory instruction. The
PRELOAD portion of this instruction is not implemented, so
the device TAP controller is not fully 1149.1 compliant.
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.
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 balls
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
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.
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.
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.
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 time (tCS plus tCH).
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; rather, it performs a capture of the I/O
ring when these instructions are executed.
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 CLK captured in the boundary scan register.
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.
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 balls.
Document #: 38-05237 Rev. *D
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