74HC238PW

The 74HC238PW from Nexperia is a high-speed Si-gate CMOS 3-to-8 line decoder/demultiplexer housed in a 16-pin TSSOP package. It is a member of the 74HC/HCT logic family, which provides pin compatibility with the older 74LS Low-Power Schottky TTL family while offering the low power consumption and wide supply voltage range of CMOS technology.

The 74HC238 decodes three binary-weighted address inputs (A0, A1, A2) to eight mutually exclusive active-HIGH outputs (Y0 to Y7). When the device is enabled (E1=LOW, E2=LOW, E3=HIGH), the output corresponding to the binary address is driven HIGH while all other outputs remain LOW. When the device is disabled (any enable condition not met), all outputs are LOW.

The key distinction between the 74HC238 and the more common 74HC138 is the output polarity: the 74HC238 has active-HIGH outputs, while the 74HC138 has active-LOW outputs. This difference is significant in memory chip-select decoding, where active-HIGH chip selects are required. Using the 74HC238 eliminates the need for inverter buffers that would otherwise be required with the 74HC138, saving board space and propagation delay.

The three enable inputs provide flexible expansion capability. Two active-LOW enables (E1, E2) and one active-HIGH enable (E3) allow four 74HC238 devices to be combined into a 5-to-32 line decoder without additional logic gates (except one inverter). Each 74HC238 handles a different 8-output segment, with the two higher-order address bits driving the enable inputs through a simple decoding scheme. This cascading capability makes the 74HC238 ideal for address decoding in memory-mapped systems.

The device can also function as a 1-to-8 demultiplexer by applying the data signal to one of the active-LOW enable inputs (E1 or E2) while using the other enable inputs as strobes. The address inputs select which output receives the data, and the remaining outputs remain LOW. This demultiplexer mode is useful for distributing clock signals, enable pulses, or serial data to one of eight destinations.

Input clamp diodes are integrated on all inputs, allowing the use of current-limiting resistors to interface inputs to voltages in excess of VCC. This feature is useful in mixed-voltage systems where input signals may come from a higher-voltage domain.

The 74HC variant operates over a wide supply voltage range (2.0V to 6.0V) with CMOS input switching levels, while the 74HCT variant operates at 4.5V to 5.5V with TTL-compatible input switching levels. Both variants share the same pinout and functional behavior, differing only in input threshold voltages. The HCT variant is recommended when interfacing with TTL or 5V CMOS logic, while the HC variant is preferred for lower-voltage and battery-powered applications.

The TSSOP-16 (PW suffix) package is the most compact through-hole/surface-mount option for this device, with a body width of 4.4mm. Other available packages include SO-16 (D suffix, 3.9mm body width), SSOP-16 (DB suffix, 5.3mm body width), and DHVQFN-16 (BQ suffix, 2.5 x 3.5mm, no leads).

The 74HC238PW operates as a combinational logic decoder that converts a 3-bit binary address to one of eight active-HIGH outputs.

Decoder Logic: The core function is implemented using AND-OR logic gates. Each output Y(n) is driven by an AND gate that takes the decoded address inputs and the enable signals as inputs. For output Y0 to be HIGH, the conditions are: A0=LOW, A1=LOW, A2=LOW, E1=LOW, E2=LOW, E3=HIGH. Similarly, Y5 is HIGH when: A0=HIGH, A1=LOW, A2=HIGH, and all enables are active. The complete decoding logic ensures that exactly one output (or none) is HIGH at any time when the device is enabled.

Address Decoding: The three address inputs (A0, A1, A2) represent a 3-bit binary number from 0 to 7. A0 is the least significant bit and A2 is the most significant bit. The output Y(n) corresponds to the decimal value of the binary address: address 000 selects Y0, address 101 selects Y5, address 111 selects Y7.

Enable Logic: The three enable inputs are combined in an AND function: Enable = NOT(E1) AND NOT(E2) AND E3. Only when all three enable conditions are satisfied (E1=LOW, E2=LOW, E3=HIGH) does the decoder activate the selected output. When the enable condition is not met, all outputs are forced LOW regardless of the address inputs. This enable logic serves multiple purposes: (1) it allows the device to be completely disabled to prevent bus contention; (2) it provides the cascading mechanism for building larger decoders; (3) it allows the device to function as a demultiplexer.

Demultiplexer Operation: To use the 74HC238 as a 1-to-8 demultiplexer, one of the active-LOW enable inputs (e.g., E1) serves as the data input, while E2 is tied LOW and E3 is tied HIGH. The address inputs select which output receives the data. When E1 is LOW (data=1), the selected output is HIGH. When E1 is HIGH (data=0), all outputs are LOW regardless of the address. Thus, the data on E1 is routed to the selected output, and the data is active-HIGH at the output (since the enable is active-LOW, inverting the data polarity).

Cascading for 5-to-32 Decoding: Four 74HC238 devices can implement a 5-to-32 line decoder using the two higher-order address bits (A3, A4) to enable the appropriate device. A simple 2-to-4 decoder (or two inverters and some NOR gates) generates the four enable signals from A3 and A4. Each 74HC238 handles eight outputs within its address range. The lower three address bits (A0, A1, A2) connect in parallel to all four devices. Only one device is enabled at a time, so only the selected output among all 32 outputs goes HIGH.

CMOS Implementation: The 74HC238 uses silicon-gate CMOS technology, with complementary N-channel and P-channel MOSFETs forming the logic gates. CMOS provides several advantages over bipolar TTL: (1) virtually zero static power dissipation (only leakage current flows when inputs are stable); (2) wide supply voltage range (2.0V to 6.0V); (3) high noise immunity due to the rail-to-rail output swing; (4) symmetrical output drive capability (+/-4mA source and sink). The propagation delay of approximately 18ns at VCC=4.5V is adequate for most address decoding applications.

Input Clamp Diodes: All inputs have integrated clamp diodes connected between the input pin and VCC/GND. These diodes limit input voltage excursions beyond the supply rails, protecting the input gates from electrostatic discharge and overvoltage. They also allow the use of series current-limiting resistors to interface with higher-voltage signals, as the clamp diodes will conduct and limit the input voltage while the resistor limits the current.