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Test data-in (TDI)
The TDI pin/ball serially inputs information into the registers and can be connected to the input of any of the registers. The register between
TDI and TDO is chosen by the instruction that is loaded into the TAP instruction register. For information on loading the instruction register,
see the TAP Controller State Diagram. TDI is internally pulled up and can be unconnected if the TAP is unused in an application. TDI is
connected to the most significant bit (MSB) of any register. (See the TAP Controller Block Diagram.)
Test data-out (TDO)
The TDO output pin/ball serially clocks data-out from the registers. The output is active depending upon the current state of the TAP state
machine. The output changes on the falling edge of TCK. TDO is connected to the least significant bit (LSB) of any register. (See the TAP
Controller State Diagram.)
Performing a TAP RESET
You can perform a RESET by forcing TMS HIGH (V
DD
) for five rising edges of TCK. This RESET does not affect the operation of the SRAM and
can be performed while the SRAM is operating.
At power-up, the TAP is reset internally to ensure that TDO comes up in a High-Z state.
TAP registers
Registers are connected between the TDI and TDO pins/balls. They allow data to be scanned into and out of the SRAM test circuitry. Only one
register can be selected at a time through the instruction register. Data is serially loaded into the TDI pin/ball on the rising edge of TCK. Data is
output on the TDO pin/ball on the falling edge of TCK.
Instruction register
You can serially load three-bit instructions into the instruction register. The register is loaded when it is placed between the TDI and TDO pins/
balls as shown in the TAP Controller Block Diagram. The instruction register is loaded with the IDCODE instruction at power up and also 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 series 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 pins/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.
Boundary scan register
The boundary scan register is connected to all the input and bidirectional pins/balls on the SRAM. Several no connect (NC) pins/balls are also
included in the scan register to reserve pins/balls for 8Mb and 18Mb Claymore SRAMs. The x36 configuration has a 70-bit-long register and
the x18 configuration has a 51-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 pins/balls when the controller is moved to the Shift-DR state. The EXTEST, SAMPLE/RELOAD, and SAMPLE Z
instructions can be used to capture the contents of the I/O ring.
The boundary scan order table shows the order in which the bits are connected. Each bit corresponds to one of the bumps on the SRAM
package. The most significant bit (MSB) of the register is connected to TDI, and the least significant bit (LSB) is connected to TDO.
Identification (ID) register
The ID register has a vendor code and other information described in the Identification Register Definitions table. 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.
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