The 'ABTH18652A and 'ABTH182652A scan test devices with 18-bit bus
transceivers and registers are members of the Texas Instruments
SCOPETM testability integrated-circuit family. This family
of devices supports IEEE Standard 1149.1-1990 boundary scan to
facilitate testing of complex circuit-board assemblies. Scan access
to the test circuitry is accomplished via the 4-wire test access port
(TAP) interface.
In the normal mode, these devices are 18-bit bus transceivers and
registers that allow for multiplexed transmission of data directly
from the input bus or from the internal registers. They can be used
either as two 9-bit transceivers or one 18-bit transceiver. The test
circuitry can be activated by the TAP to take snapshot samples of the
data appearing at the device pins or to perform a self test on the
boundary-test cells. Activating the TAP in the normal mode does not
affect the functional operation of the SCOPETM bus
transceivers and registers.
Data flow in each direction is controlled by clock (CLKAB and
CLKBA), select (SAB and SBA), and output-enable (OEAB and ) inputs. For A-to-B data flow,
data on the A bus is clocked into the associated registers on the
low-to-high transition of CLKAB. When SAB is low, real-time A data is
selected for presentation to the B bus (transparent mode). When SAB
is high, stored A data is selected for presentation to the B bus
(registered mode). When OEAB is high, the B outputs are active. When
OEAB is low, the B outputs are in the high-impedance state. Control
for B-to-A data flow is similar to that for A-to-B data flow, but
uses CLKBA, SBA, and
inputs. Since the input is
active-low, the A outputs are active when is low and are in the
high-impedance state when is
high. Figure 1 illustrates the four fundamental bus-management
functions that are performed with the 'ABTH18652A and 'ABTH182652A.
In the test mode, the normal operation of the SCOPETM
bus transceivers and registers is inhibited, and the test circuitry
is enabled to observe and control the I/O boundary of the device.
When enabled, the test circuitry performs boundary-scan test
operations according to the protocol described in IEEE Standard
1149.1-1990.
Four dedicated test pins observe and control the operation of the
test circuitry: test data input (TDI), test data output (TDO), test
mode select (TMS), and test clock (TCK). Additionally, the test
circuitry performs other testing functions such as parallel-signature
analysis (PSA) on data inputs and pseudo-random pattern generation
(PRPG) from data outputs. All testing and scan operations are
synchronized to the TAP interface.
Improved scan efficiency is accomplished through the adoption of a
one boundary-scan cell (BSC) per I/O pin architecture. This
architecture is implemented in such a way as to capture the most
pertinent test data. A PSA/COUNT instruction is also included to ease
the testing of memories and other circuits where a binary count
addressing scheme is useful.
Active bus-hold circuitry holds unused or floating data inputs at
a valid logic level.
The B-port outputs of 'ABTH182652A, which are designed to source
or sink up to 12 mA, include 25- series resistors to reduce overshoot and undershoot.
The SN54ABTH18652A and SN54ABTH182652A are characterized for
operation over the full military temperature range of -55°C to
125°C. The SN74ABTH18652A and SN74ABTH182652A are characterized
for operation from -40°C to 85°C.
