IDT JTAG Boundary Scan
Instruction Register (IR)
79RC32438 User Reference Manual
19 - 7
November 4, 2002
Notes
SAMPLE/PRELOAD
The sample/preload instruction has a dual use. The primary use of this instruction is for preloading the
boundary scan register prior to enabling the EXTEST instruction. Failure to preload will result in unknown
random data being driven onto the output pins when EXTEST is selected. The secondary function of
SAMPLE/PRELOAD is for sampling the system state at a particular moment. Using the SAMPLE function,
the user can halt the device at a certain state and shift out the status of all of the pins and output enables at
that time.
BYPASS
The BYPASS instruction is used to truncate the boundary scan register to a single bit in length. During
system level use of the JTAG, the boundary scan chains of all the devices on the board are connected in
series. In order to facilitate rapid testing of a given device, all other devices are put into BYPASS mode.
Therefore, instead of having to shift 499 times to get a value through the RC32438, the user only needs to
shift one time to get the value from JTAG_TDI to JTAG_TDO. When the TAP controller passes through the
CAPTURE-DR state, the value in the BYPASS register is updated to be 0.
If the device being used does not have a DEVICE_ID register, then the BYPASS instruction will automat-
ically be selected into the instruction register whenever the TAP controller is reset. Therefore, the first value
that will be shifted out of a device without a DEVICE_ID register is always 0. Devices such as the RC32438
that include a DEVICE_ID register will automatically load the DEVICE_ID instruction when the TAP
controller is reset, and they will shift out an initial value of 1. This is done to allow the user to easily distin-
guish between devices having DEVICE_ID registers and those that do not.
CLAMP
This instruction, listed as optional in the IEEE 1149.1 JTAG Specifications, allows the boundary scan
chain outputs to be clamped to fixed values. When the clamp instruction is issued, the scan chain will
bypass the RC32438 and pass through to devices further down the scan chain.
DEVICEID
The DEVICEID instruction is automatically loaded when the TAP controller state machine is reset either
by the use of the JTAG_TRST_N signal or by the application of a ‘1’ on JTAG_TMS for five or more cycles
of JTAG_TCK as per the IEEE Std 1149.1 specification. The least significant bit of this value must always
be 1. Therefore, if a device has a DEVICE_ID register, it will shift out a 1 on the first shift if it is brought
directly to the SHIFT-DR TAP controller state after the TAP controller is reset. The board- level tester can
then examine this bit and determine if the device contains a DEVICE_ID register (the first bit is a 1), or if the
device only contains a BYPASS register (the first bit is 0).
However, even if the device contains a DEVICE_ID register, it must also contain a BYPASS register. The
only difference is that the BYPASS register will not be the default register selected during the TAP controller
reset. When the DEVICE_ID instruction is active and the TAP controller is in the Shift-DR state, the thirty-
two bit value that will be shifted out of the device-ID register is
0x00022067
.
Bit(s)
Mnemonic
Description
R/W
Reset
0
reserved
reserved 0x1
R
1
11:1
Manuf_ID
Manufacturer Identity
(11 bits)
IDT 0x33
R
0x33
27:12
Part_number
Part Number
(16 bits)
This field identifies the part number of the processor derivative.
For the RC32438 this value is: 0x0022
R
impl.
dep.
31:28
Version
Version
(4 bits)
This field identifies the version number of the processor derivative.
For the RC32438, this value is 0x0
R
impl.
dep.
Table 19.3 System Controller Device Identification Register