Reinhardt Wagner, Maxim Integrated Products
This Design Idea describes a simple alternative to a nonvolatile gating function you typically implement using PAL (programmable-array logic), GAL (gate-array logic), or a CPLD (complex-programmable-logic device). To gate a logic signal to block or transmit it, you usually employ a logic gate, such as an AND gate, and use the gate’s second input to define whether the gate blocks or transmits the applied signal. Because logic gates perform immediate Boolean operations, their operations are combinational and without memory.
However, if you must program a gate that should always either block or transmit the signal after system start-up, you must store the “transmit/block” logic state in some form of nonvolatile memory. Two basic methods are available for storing such logic states. The first involves using a microcontroller in combination with nonvolatile memory, such as EEPROM. This method is suitable if the system can wait until the microcontroller reads the logic state from memory and applies it to a hardware pin – typically, through a general-purpose I/O pin. Some systems, however, require that the transmit/block signal be present at start-up. For those systems, the read delay from memory is unacceptable.
A second method, which is useful for systems without a microcontroller or that cannot wait for the microcontroller to read from memory at boot time, stores the logic state in a device that makes it immediately available at power-up. For this purpose, PAL devices, GAL devices, and CPLDs implement the gating function in combination with programmable nonvolatile memory. These devices offer more than gating with memory, however, and may be overspecified for systems that need only a few such gates. Also, their packages are relatively large to accommodate the many logic-I/O pins they offer.
If you need only a few nonvolatile gates, consider using a component common in analog- and mixed-signal systems: the digital potentiometer (Figure 1). Ground the L end of the resistor string and route the signal into the H end of the string. Then, the wiper output either shorts to ground for blocking or connects to the input signal for transmission.
|Figure 1.||A programmable, nonvolatile digital potentiometer functions
as a simple AND gate. Setting the wiper to the device’s highest
value allows the input signal to propagate to the output; setting
the wiper to the lowest value blocks the input signal.
You can program the digital potentiometer through its serial interface during board or system test. The up/down interface on some digital potentiometers is suitable for that purpose. When selecting a nonvolatile digital potentiometer, you should consider the following criteria:
The digital potentiometer’s bandwidth determines the maximum data rate for signals transmitted through the potentiometer. If the switching rate of these applied logic signals is too high for the available potentiometers, you can use a conventional, high-speed logic gate with a digital potentiometer controlling the transmit/block input (Figure 2).
|Figure 2.||If the bandwidth of the digital potentiometer is too low,
you can use the device to drive an AND gate.
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