74HCT245PW

The 74HCT245PW from Nexperia is an octal (8-bit) bidirectional bus transceiver with non-inverting 3-state outputs, designed for asynchronous two-way communication between data buses. It is one of the most widely used standard logic devices in digital system design, serving as the primary interface between processors, memory, and peripheral devices on shared bus architectures.

The 74HCT245 belongs to the 74HC/HCT family of high-speed silicon-gate CMOS devices. The HCT variant features TTL-compatible input voltage levels (VIH minimum 2.0V at VCC=4.5V), making it ideal for interfacing between TTL and CMOS logic families. This compatibility allows the 74HCT245 to accept signals from TTL-level microprocessors, ASICs, and FPGAs while driving CMOS-level loads, without requiring external level-shifting circuitry.

The device provides two control inputs that manage bus operation. The output enable (OE) input, active LOW, controls whether the outputs are active or in the high-impedance state. When OE is HIGH, all outputs assume a high-impedance (3-state) condition, effectively disconnecting the device from both buses. This allows multiple 74HCT245 devices to share the same bus in a wired-OR configuration without bus contention. The direction (DIR) input determines the data flow direction: when DIR is LOW, data flows from the A bus to the B bus; when DIR is HIGH, data flows from the B bus to the A bus.

The 3-state output capability is essential for bus-oriented systems. In a typical microprocessor system, multiple devices (memory, I/O controllers, DMA controllers) share a common data bus. Only one device can drive the bus at any given time. The 3-state outputs of the 74HCT245 allow a device to be electrically disconnected from the bus when it is not actively driving data, preventing bus contention and signal corruption.

The non-inverting characteristic means that the data passes through the transceiver without inversion: a HIGH on the input appears as a HIGH on the output. This simplifies system design by eliminating the need to account for signal inversion in the bus interface logic. For applications requiring inversion, the 74HCT640 (inverting version) is available in the same pinout.

The propagation delay of 10ns typical at VCC=5V makes the 74HCT245 suitable for moderate-speed bus systems. The output drive capability of plus/minus 6mA at 5V can directly drive bus lines or up to 15 LSTTL loads, providing adequate fan-out for most applications. The balanced output design ensures symmetrical rise and fall times, minimizing signal skew on the bus.

The TSSOP-20 (PW) package offers a compact footprint compared to the traditional SOIC-20 (D) package, making it suitable for space-constrained designs such as portable equipment and dense circuit boards. The 0.65mm pin pitch is compatible with standard PCB manufacturing processes.

The device includes positive input clamp diodes on all inputs, which limit negative voltage excursions below GND. This allows the use of current-limiting resistors to interface inputs to voltages in excess of VCC, enabling direct connection to higher-voltage bus signals without damage. The latch-up performance exceeds 100mA per JESD78 Class II, ensuring robust operation in electrically noisy environments.

The 74HCT245 is part of a large family of bus transceivers with various configurations. Related devices include the 74HC245 (CMOS input levels, wider VCC range of 2-6V), 74LVC245 (lower voltage, 1.65-3.6V), 74LVT245 (2.7-3.6V with higher drive), and 74ABT245 (5V with very high drive). The 74HCT245 occupies the sweet spot for 5V systems that need TTL-compatible inputs with CMOS-level outputs.

The 74HCT245PW operates as an 8-bit bidirectional bus transceiver using two sets of 3-state buffers controlled by OE and DIR inputs.

Internal Architecture: The device contains 16 3-state buffers organized as 8 pairs. Each pair connects one A-bus line to one B-bus line. One buffer in each pair drives from A to B, and the other drives from B to A. The OE and DIR control inputs select which set of buffers is enabled, or whether all buffers are disabled.

Control Logic: The control logic decodes the OE and DIR inputs as follows:
1. OE=HIGH, DIR=X: All outputs are in the high-impedance (3-state) condition. Both the A and B bus pins are effectively disconnected from the internal circuitry. This allows other devices to drive the bus without contention.
2. OE=LOW, DIR=LOW: The A-to-B buffers are enabled. Data present on the A0-A7 pins is transmitted through the non-inverting buffers to the B0-B7 pins. The A bus pins are configured as inputs and the B bus pins are outputs.
3. OE=LOW, DIR=HIGH: The B-to-A buffers are enabled. Data present on the B0-B7 pins is transmitted through the non-inverting buffers to the A0-A7 pins. The B bus pins are configured as inputs and the A bus pins are outputs.

3-State Output Stage: Each output buffer has a 3-state output stage consisting of a PMOS pull-up transistor and an NMOS pull-down transistor in a push-pull configuration, plus a disable circuit that turns off both transistors. When the output is active, one transistor is on and the other is off, driving the output HIGH or LOW. When the output is in the high-impedance state, both transistors are off, and the output pin presents a very high impedance to the bus (only leakage current flows). The output enable circuitry ensures that the disable transition happens cleanly, with the output going to high-impedance before any other device begins driving the bus.

TTL-Compatible Input Stage (HCT): The HCT variant uses a modified input stage that responds to TTL voltage levels. In standard CMOS (HC), the input switching threshold is approximately VCC/2 (2.5V at 5V). In the HCT variant, the switching threshold is shifted lower to approximately 1.4V, matching the TTL specification. This is achieved by adjusting the P/N ratio of the input inverter and adding a level-shifting circuit. The HCT input accepts VIH as low as 2.0V (vs. 3.5V for HC at 5V) and VIL as high as 0.8V, directly compatible with TTL output specifications (VOH min 2.4V, VOL max 0.4V).

Bus Contention Prevention: In systems with multiple bus transceivers, the OE signal is used to ensure that only one device drives the bus at any time. The bus controller typically uses address decoding logic to generate individual OE signals for each transceiver. The turn-off time of the 74HCT245 (typically 10ns) ensures that the current driver releases the bus quickly enough for the next driver to take over without a significant dead time.

Power Dissipation: The static power consumption of the HCT variant is higher than the HC variant due to the additional DC current drawn by the TTL-compatible input stage when driven with TTL-level signals. The additional supply current is approximately 400uA per input at VCC=4.5V to 5.5V when driven with VIH=VCC-2.1V. The dynamic power consumption is determined by the load capacitance and switching frequency: PD = CPD x VCC squared x fi x N, where CPD is the power dissipation capacitance per buffer.