
CMOS
MT90820
17
JTAG Support
The MT90820 JTAG interface conforms to the
Boundary-Scan standard IEEE1149.1. This standard
specifies a design-for-testability technique called
Boundary-Scan test (BST). The operation of the
boundary-scan circuitry is controlled by an external
test access port (TAP) Controller.
Test Access Port (TAP)
The Test Access Port (TAP) provides access to the
many test functions of the MT90820. It consists of
three input pins and one output pin. The following
pins are from the TAP.
Test Clock Input (TCK)
TCK provides the clock for the test logic. The
TCK does not interfere with any on-chip clock
and thus remain independent. The TCK permits
shifting of test data into or out of the
Boundary-Scan register cells concurrently with
the operation of the device and without
interfering with the on-chip logic.
Test Mode Select Input (TMS)
The logic signals received at the TMS input are
interpreted by the TAP Controller to control the
test operations. The TMS signals are sampled
at the rising edge of the TCK pulse. This pin is
internally pulled to Vdd when it is not driven
from an external source.
Test Data Input (TDI)
Serial input data applied to this port is fed
either into the instruction register or into a test
data register, depending on the sequence
previously applied to the TMS input. Both
registers are described in a subsequent
section. The received input data is sampled at
the rising edge of TCK pulses. This pin is
internally pulled to Vdd when it is not driven
from an external source.
Test Data Output (TDO)
Depending on the sequence previously applied
to the TMS input, the contents of either the
instruction register or data register are serially
shifted out towards the TDO. The data out of
the TDO is clocked on the falling edge of the
TCK pulses. When no data is shifted through
the boundary scan cells, the TDO driver is set
to a high impedance state.
Test Reset (TRST)
Reset the JTAG scan structure. This pin is
internally pulled to VDD.
Instruction Register
In accordance with the IEEE 1149.1 standard, the
MT90820 uses public instructions. The MT90820
JTAG Interface contains a two-bit instruction register.
Instructions are serially loaded into the instruction
register from the TDI when the TAP Controller is in
its shifted-IR state. Subsequently, the instructions
are decoded to achieve two basic functions: to select
the test data register that may operate while the
instruction is current, and to define the serial test
data register path, which is used to shift data
between TDI and DO during data register scanning.
Test Data Register
As specified in IEEE 1149.1, the MT90820 JTAG
Interface contains two test data registers:
The Boundary-Scan register
The Boundary-Scan register consists of a
series of Boundary-Scan cells arranged to form
a scan path around the boundary of the
MT90820 core logic.
The Bypass Register
The Bypass register is a single stage shift
register that provides a one-bit path from TDI to
its TDO.
The MT90820 boundary scan register contains 118
bits. Bit 0 in Table 15 Boundary Scan Register is the
first bit clocked out. All tristate enable bits are active
high.