Bushold functions have been added to Fairchild Semiconductor's VCX portfolio of 16-bit buffer/line drivers, transparent latches, and flip-flops. Developed by the company's Interface and Logic Group, South Portland, Maine, this feature adds more versatility to the 2.5-V CMOS logic family.
This new technology maintains a valid logic state on an unloaded input. In turn, external-conditioning pull-up or pull-down resistors don't need to prevent floating inputs. These devices also have the fastest CMOS logic available at rise times of less than 2 ns (at 3 V), aiding in high-performance, low-voltage designs.
Typically, the bushold acts like a very weak clamp at the output. It maintains the floating bus in its last-known state. Since it is weak, it requires only a relatively small current to keep the bus in the required state. The bushold also doesn't need much current to overcome the clamp once anything else is activated on the bus.
In bus applications, different clients have the potential to enter tristate conditions in a bidirectional fashion at any given time. Consequently, it's difficult to determine which component controls the bus. Bushold, then, is a really useful feature. It consumes some additional power, so using VCX devices without the bushold option increases their flexibility.
The VCX series is particularly well suited for distributed-bus applications, such as driving memory modules, memory arrays, or other kinds of peripheral bus structures. Additional uses include high-performance servers, workstations and other computing systems, DIMM modules, high-speed graphics, and high-speed networking applications. Devices in the series also are useful as plain buffers.
Excessive use of bushold devices can require additional current to switch the bus. Only one bushold device is required to hold the last state of the bus as well. That's why Fairchild also offers the VCXH series of devices, which don't have a bushold. Designers can use these to minimize the power supply's drive requirements.
The 16-bit, low-voltage VCXH line features the 74VCXH16240 inverting buffer/line driver, the 74VCXH162240 inverting buffer/line driver with 26-Ω series resistors in the outputs, and the 74VCXH16244 buffer/line driver. Other models include the 74VCXH162244 buffer/line driver with 26-Ω series resistors in the outputs, the 74VCXH16373 transparent latch, and the 74VCXH162376 transparent latch with 26-Ω series resistors in the outputs. The 74VCXH16374 D-type flip-flop and the 74VCXH162374 D-type flip-flop with 26-Ω series resistors in the outputs are available, too.
As with all of the members of the VCX portfolio and their bushold counterparts, the 16-bit VCXH devices offer a wide voltage operating range. Designers can interface to either 2.5-V, mixed 2.5-/3.3-V, or 1.8-V systems. The VCX series, meanwhile, comes in TSSOPs and SOIC packages. Devices cost $1.49 each in 1000-piece quantities. Samples and production quantities are available.
Fairchild Semiconductor, 333 Western Ave., South Portland, MN 04106; (888) 522-5372; fax (972) 910-8036; www.fairchildsemi.com.