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Logic - Shift Registers

The SN74F299NSRE4, offered by Texas Instruments, is a 16-bit Serial-In/Serial-Out Shift Register with 3-state Outputs. It has two independent 8-bit registers with separate clock and serial input pin, enabling them to operate as two independent 8-bit shift registers, or as one 16-bit shift register. SH/LD pin is used to select the operation mode, either "Shift Mode" or "Load Mode". It has an asynchronous clear pin which enables the instantaneous clearing of the register, and three 3-state outputs which enable the register to be used as an address/data multiplexer.

Application Field

The SN74F299NSRE4 is typically used in a number of applications, such as:

• Fabricating shift registers
• Adder and subtracter
• Signal multiplexing
• Address/data multiplexing
• Address/data demultiplexing
• Frequency division
• Time division multiplexing

Working Principle

The SN74F299NSRE4 can operate in two modes, the Shift Mode and the Load Mode. The SH/LD pin is used to select the mode.

In "Shift Mode", the contents of the registers are shifted by one position from either the left or the right. A clock signal is applied at the clock input pin, upon which the data present at the serial input pin is shifted in therelevant register. If the clock signal is of falling edge triggered type, the data is shifted toward the MSB and if it is of rising edge triggered type, the data is shifted toward the LSB.

In "Load Mode" the contents of the register are loaded simultaneously from the serial input pin(s). As there are two registers with separate synchronous loading, they can be loaded asynchronously as well.

The SN74F299NSRE4 also has 3 3-state outputs which are used to drive an external load. These three-state outputs can be enabled/disabled by the ENABLE signal. The ENABLE signal can be group generated for multiple devices.

The SN74F299NSRE4 has an asynchronous Clear pin which enables the instantaneous clearing of the registers. This signal is independent of the logic levels applied to the other input pins.

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