40MX and 42MX FPGA Families
Revision 11 1-13
Design Consideration
It is recommended to use a series 70Ω termination resistor on every probe connector (SDI, SDO, MODE,
DCLK, PRA and PRB). The 70Ω series termination is used to prevent data transmission corruption
during probing and reading back the checksum.
IEEE Standard 1149.1 Boundary Scan Test (BST) Circuitry
42MX24 and 42MX36 devices are compatible with IEEE Standard 1149.1 (informally known as Joint
Testing Action Group Standard or JTAG), which defines a set of hardware architecture and mech anisms
for cost-effective board-level testing. The basic MX boundary-scan logic circuit is composed of the TA P
(test access port), TAP controller, test data registers and instruction register (Fi gure 1-13 on page 1-14).
This circuit supports all mandatory IEEE 1149.1 instructions (EXTEST, SAMPLE/PRELOAD and
BYPASS) and some optional instructions. Table 1-3 on page 1-14 describes the ports that control JTAG
testing, while Table 1-4 on page 1-14 describes the test instructions supported by these MX devices.
Each test section is accessed through the TAP, which has four associated pins: TCK (test clock input),
TDI and TDO (test data input and output), and TMS (test mode selector).
The TAP controller is a four-bit state machine. The '1's and '0's represent the values that must be
present at TMS at a rising edge of TCK for the given state transition to occur. IR and DR indicate that the
instruction register or the data register is operating in that state.
The TAP controller receives two control i nputs (TMS and TCK) and generates control and clock sign als
for the rest of the test logic architecture. On power-up, the TAP controller enters the Test-Logic-Reset
state. To guarantee a reset of the controller from any of the possible states, TMS must remain high for
five TCK cycles.
42MX24 and 42MX36 devices support three types of test data registers: bypass, device identification,
and boundary scan. The bypass register is selected when no other register needs to be accessed in a
device. This speeds up test data transfer to other devices in a test data path. The 32-bit device
identification register is a shift register with four fields (lowest significant byte (LSB), ID number, part
number and version). The boundary-scan register ob serves and controls the state of each I/O pin.
Each I/O cell has three boundary-scan register cells, each with a serial-in, serial-out, parallel-in, and
parallel-out pin. The serial pins are used to serially connect all the boundary-scan register cells in a
device into a boundary-scan register chain, which starts at the TDI pin and ends at the TDO pin. The
Table 1-2 • Device Configuration Options for Probe Capabili ty
Security Fuse(s)
Programmed Mode PRA, PRB
1
SDI, SDO, DCLK
1
No LOW User I/Os
2
User I/Os
2
No HIGH Probe Circuit Outputs Probe Circuit Inputs
Yes – Probe Circuit Secured Probe Circuit Secured
Notes:
1. Avoid using SDI, SDO, DCLK, PRA and PRB pins as input or bidirectional ports. Since these pins are
active during probing, input signals will not pass through these pins and may cause contention.
2. If no user signal is assigned to these pins, they will behave as unused I/Os in this mode. See the "Pin
Descriptions" section on page 1-83 for information on unused I/O pins.
