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TPMC632 User Manual Issue 1.0.6
Page 29 of 49
I/O Interface
6.2
6.2.1 TTL I/O Interface
Each of the 64 (TPMC632x0) or 32 (TPMC632x2) TTL I/O line contains two 74LVT126 bus buffers as an
interface to the FPGA pins. The logic levels of the buffers are TTL compatible, meaning that the minimum
high level is 2.0V and the maximum low level is 0.8V. The nominal output high voltage is 3.3V.
The buffer outputs are followed by 47
Ω
serial resistors for signal integrity reasons. The 4.7k
Ω
pull up
resistors guaranty a high level when outputs are tristate and not driven externally.
As an option, the pull up voltage can be set to 5V by jumper J1 to (weakly) drive a higher voltage than 3.3V.
This means, instead of toggling the corresponding FPGA_OUT I/O pin, the corresponding FPGA_OE pin
(output enable) can be used, to pull the line low or set it in tristate to obtain a high-level. For example when
connecting to a standard 5V CMOS logic input (not TTL compatible levels), a high level of minimum 3.5V is
required.
Please note that the pull up resistors can only drive high impedance inputs.
A TVS array protects against ESD shocks.
See the following figure for more information of the TTL I/O circuitry.
Please note that the length (and consequently the capacitance) of a flat cable, connected to the
TPMC632 module, should be kept as short as possible to prevent large cross talk.
To reduce the cross talk on the TPMC632, not all 64 I/O lines should be switched at the same
time. For example, the output lines should be switched in groups of 8 signals in steps of 12ns,
meaning that after about 100ns the switching process is completed.
XILINX
FPGA
FPGA_OUTx
FPGA_OEx
FPGA_INx
4k7
74LVT126
1 M-LVDS Line
X1 / P14
47R
5V, 3.3V or GND
3.3V
MSMF05
(Protection)
Figure 6-2 : TTL I/O Interface