The SN74GTL16616 is a 17-bit UBT transceiver that provides LVTTL-to-GTL/GTL+ and GTL/GTL+-to-LVTTL signal-level translation.
Combined D-type flip-flops and D-type latches allow for transparent, latched, clocked, and clocked-enabled
modes of data transfer identical to the '16601 function. Additionally, this device provides for a copy of CLKAB
at GTL/GTL+ signal levels (CLKOUT) and conversion of a GTL/GTL+ clock to LVTTL logic levels (CLKIN). This
device provides an interface between cards operating at LVTTL logic levels and a backplane operating at
GTL/GTL+ signal levels. Higher-speed operation is a direct result of the reduced output swing (<1 V), reduced
input threshold levels, and OEC circuitry.
The user has the flexibility of using this device at either GTL (VTT = 1.2 V and VREF = 0.8 V) or the preferred
higher noise margin GTL+ (VTT = 1.5 V and VREF = 1 V) signal levels. GTL+ is the Texas Instruments derivative
of the Gunning Transceiver Logic (GTL) JEDEC standard JESD 8-3. The B port normally operates at GTL or
GTL+ signal levels, while the A-port and control inputs are compatible with LVTTL logic levels and are 5-V
tolerant. V REF is the reference input voltage for the B port. VCC (5 V) supplies the internal and GTL circuitry while
VCC (3.3 V) supplies the LVTTL output buffers.
Data flow in each direction is controlled by output-enable (OEAB\ and OEBA\), latch-enable (LEAB and LEBA),
and clock (CLKAB and CLKBA) inputs. The clock can be controlled by the clock-enable (CEAB\ and CEBA\)
inputs. For A-to-B data flow, the device operates in the transparent mode when LEAB is high. When LEAB is
low, the A data is latched if CEAB\ is low and CLKAB is held at a high or low logic level. If LEAB is low, the A-bus
data is stored in the latch/flip-flop on the low-to-high transition of CLKAB if CEAB\ also is low. When OEAB\ is
low, the outputs are active. When OEAB\ is high, the outputs are in the high-impedance state. Data flow for B
to A is similar to that of A to B, but uses OEBA\, LEBA, CLKBA, and CEBA\.
This device is fully specified for partial-power-down applications using Ioff. The Ioff circuitry disables the outputs,
preventing damaging current backflow through the device when it is powered down.
Active bus-hold circuitry holds unused or undriven LVTTL inputs at a valid logic state. Use of pullup or pulldown
resistors with the bus-hold circuitry is not recommended.
To ensure the high-impedance state during power up or power down, OE\ should be tied to VCC through a pullup
resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver.
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