Cypress Perform CY7C1380F - Manual (Page 13)

CY7C1380D, CY7C1382D
CY7C1380F, CY7C1382F
Document #: 38-05543 Rev. *F Page 13 of 34
When the TAP controller is in the Capture-IR state, the two least
significant bits are loaded with a binary ‘01’ pattern to enable 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 enables 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 16.
TAP Instruction Set
Overview
Eight different instructions are possible with the three bit
instruction register. All combinations are listed in “Identification
Codes” on page 16. Three of these instructions are listed as
RESERVED and must not be used. The other five instructions
are described in detail in this section.
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 must 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 enables
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 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 undergoes a transition. The TAP may then try to
capture a signal while in transition (metastable state). This does
not harm the device, but there is no guarantee as to the value
that is 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 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 enables 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 is 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 balls. The advantage of the
BYPASS instruction is that it shortens the boundary scan path
when multiple devices are connected together on a board.
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 #85
(for 119-BGA package) or Bit #89 (for 165-fBGA package). When
this scan cell, called the “extest output bus tri-state,” is latched
into the preload register during the Update-DR state in the TAP
controller, it directly controls the state of the output (Q-bus) pins,
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