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Introduction Designers today are often faced with system requirements that necessitate the use of components using a variety of supply voltages. It is not uncommon for a circuit board to contain components requiring 5.0 volts and additional components requiring 3.3 volts, 2.5 volts, or sometimes even a "non-standard" voltage. Some examples of components requiring 2.5V or "non-standard" voltages are DSP Cores, ASICs, and Programmable Logic. Designers are already developing systems that will require even lower voltages (1.8 volts and below).

Summit Microelectronics, Inc. has developed a versatile family of devices that monitor both voltages of these dual supply systems, regardless of the combination of voltages.

Background: Why Lower Voltages?
Lower operating voltages allow designers to develop circuits that require less power, operate faster, and generate less noise. The semiconductor industry rapidly develops processes with smaller geometries in a constant effort to lower die sizes (hence lower costs), increase the amount of circuitry (hence higher integration and higher functionality), and improve performance (higher speed and lower power requirements). An additional benefit is realized in that the device supply voltage drops with every new process geometry shrink. At a 0.5 micron process, a semiconductor device’s transistors operate at 5.0 volts. That same device using a 0.35 micron process operates at 3.3 volts. Each successive shrink in the process requires a new voltage. Figure 1 shows this progression.

 Figure 1. Voltage Migration of Semiconductor Processes

Overview
The S42xx family of Supervisory Circuits can monitor two supply voltages and generate the desired Reset or other notification function(s). The S42xx supervisory device will operate from one of the available voltages, provided that this voltage falls within the range of 2.7V to 5.5V. The S42xx features a second sensor that will monitor the second supply voltage provided the minimum level is above 1.24V.

The S42xx family features complimentary RESET outputs for the supply voltage monitoring function. The device also features a single, active low output, V_LOW# for the output signal associated with the second supply voltage. The RESET and RESET# pins are actually I/O's on the S42xx, increasing the versatility of the device. Some of the applications options for these three indicator lines are detailed later in this applications note.

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Voltage Monitoring
The supply voltage, V_CC, for the S42xx is monitored by comparing the actual level against a threshold voltage, V_TRIP. All Summit supervisory devices, including the S42xx, feature a programmable V_TRIP so that the designer can determine the optimal threshold level for his circuit. This level is usually determined by evaluating the minimum reliable operating ranges of all of the active devices in the design. The highest of these values represents the "worst case" operating condition and becomes the optimum V_TRIP. Once the designer determines the optimum V_TRIP, Summit can provide the S42xx with the desired V_TRIP to within 50mV accuracy for 5 volt systems and 30mV accuracy for 3 volt systems. A low cost programmer is also available from Summit for on-sight or in-circuit programming. This represents a logistical improvement over the previous generation of reset controllers in that a manufacturer can purchase one basic product and program it to fit the unique requirements of multiple assemblies.

The second supply voltage is monitored using the S42xx's V_SENSE input. The V_SENSE input compares the input level against an internal precision 1.24V reference voltage. The designer then determines the threshold level where the device should trip when the second voltage source decays. An external voltage divider is used to divide down this voltage to 1.24V at V_SENSE. For example, if the desired threshold point is 2.7V (in a 3V system, for example), then the values of R1 is 11.5KW ±1% and R2 is 10.0KW ±1% (see Figure 2). Thus, when the 3V supply drops below 2.7V, V_DIV drops below 1.24V, tripping the V_SENSE input. Figure 3 shows an example using the S42xx to monitor a system with a 5V supply for logic and a 2.5V supply for programmable logic. Figure 3 assumes a V_TRIP of 2.25V max

Output Options
When V_CC decays and drops below V_TRIP, the RESET and RESET# signals are asserted. For further discussion of the value of complementary RESET signals, see Summit Applications Note 10, Using Summit Reset Controllers in Multi-Microcontroller Systems. The typical application of these signals is the direct Reset of the microcontroller, ASIC, chipset, etc.

When the V_SENSE input is tripped, the corresponding output, V_LOW#, is activated. V_LOW# can be tied to an interrupt line or a port pin and processed separately from the RESET# output which trips from the monitoring of V_CC (as in Figure 2). If a single Reset signal for both voltages is desired, the S42xx device allows this as well. Both RESET and RESET# are actually I/O's. Both V_LOW# and RESET# outputs are active-low and open-drain. Tying RESET# and V_LOW# together (wired-OR) gives the designer a single Reset signal that trips when either of the two supply voltages drops below the desired threshold (see Figure 4). The standard Reset timout period, t_PURST, applies.

An external signal (manual reset switch, master Reset signal, etc.) can also be tied to one of these pins to generate a reset input signal (see Figure 5). This input will trigger both RESET and RESET# outputs. In the dual-voltage Reset example, the V_LOW#, wired-OR'ed with RESET#, will activate both RESET and RESET#. The standard Reset timeout period, t_PURST, applies.

Other Uses for V_SENSE
The V_SENSE input can be used to monitor more than a second supply voltage. One example is using the sensor to monitor a battery voltage. When the battery voltage decays to a pre-determined level, the S42xx can provide an interrupt signal that the processor can use to take appropriate action, such as enabling a battery charging circuit.

V_SENSE can also be used to monitor the unregulated DC in a system. This gives the user an early power fail indicator. This is especially useful to prevent data corruption in nonvolatile memories when housekeeping routines are executed prior to power-down. See Applications Note 013: Preventing Data Corruption During Power Failures.

Other Features and Options
The S42xx family also features an on-board I²C E² memory. This memory is 100% software compatible to the industry standard 24Cxx family of serial E²PROM devices. This eliminates the need for the separate E² device, saving cost and board space. In addition, the S42xx internal V_CC monitor circuitry protects the memory array from data corruption, improving the overall system reliability.

This family of product also has a watchdog timer function available. The S42WDxx devices feature a 1.6 second nominal timer (programmable from 400mS to 6.4S) that is reset via the I²C interface. If not reset before the watchdog interval, the RESET and RESET# signals are activated.

Summary
The S42xx family of Supervisory Circuits provides the user with a single-chip solution for monitoring both voltages in dual-voltage systems. The output structure of the Summit device gives the user the option of generating separate low voltage indicators or a common indicator (Reset). The additional sense function uses a precision 1.24V reference so that any voltage above this value can be monitored. This, coupled with the programmable trip point of the V_CC monitor, allows the S42xx to be optimized to the unique requirements of the individual design.

The S42xx has other features that increase the reliability of the overall design and offers cost savings and board space savings by the higher level of integration. This includes the external reset (or manual reset switch) input feature and the integrated E² memory array.

The following table gives examples of Summit device and resistor values for common supply voltage combinations:

 

 

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