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Introduction Designers of embedded control systems regularly take advantage of the benefits offered by Summit's Precision Reset Controller family as reset control is a necessary and important element of reliable microcontroller design. The overall function of the reset control, or supervisory circuit, and the serial E2PROM memory are understood and the improvements offered by the Summit device are summarily recognized. This recognition does not come quite as easily to engineers who design with chipsets rather than common microcontrollers.

This applications note will explain how using the Summit S24022 Precision Reset Controller can enhance modem designs based on the TI X2 chipset.

X2 Requirements
The X2 chipset processor requires an external precision reset control device, or supervisory circuit, to provide a reliable reset signal to the chipset processor. This ensures proper modem operation by

• 1) controlling the critical voltage conditions on power up,
• 2) protecting the system from improper operation during brown-out situations, and
• 3) allowing the modem to power-down in a controlled manner.
  

Of all these functions, the power-up aspect is probably the most critical to modem designers.

Reset control has long been a critical aspect of embedded control design and many semiconductor products have been introduced to perform this function reliably. Virtually every microcontroller design now utilizes an external precision reset control circuit. Upon power up, proper reset control ensures that the environment (power, external system, etc.) is stable before the microcontroller is allowed to perform its initialization routines. Failure to do this may result in unreliable operation or system lock-up. The reset control function also protects the system during power-down and supply voltage brown-out situations. A single bit misinterpreted due to marginal thresholds could cause many problems, including unintended jumps to executable routines, potentially wreaking havoc in the overall system. Data corruption of the nonvolatile memory (93C66 serial E²PROM for X2 designs) is also a common result of these situations. For these reasons, the use of discreet precision reset controllers has become a standard element of embedded control design.

Designers of PC add-in cards face these same issues plus more worrisome problems relating to the host interface. Inadequate reset from the host machine (and some platforms often give the card no detectable reset at all) may cause the chipset processor to latch-up, a potentially catastrophic situation! In addition, the slew rate of V_CC upon system power-up can vary greatly from platform to platform, from machine to machine within a given platform, and can vary significantly within the same machine. Glitches or perturbations in the V_CC, particularly in the range of the chipset processors operating threshold can potentially lock-up the processor or, much worse, cause device latch-up. It is for these reasons that the reset control function is required for X2 modem designs.

Unfortunately, reset devices have traditionally been designed for microcontroller environments where the system is closed and the reset function can be implemented by monitoring the supply voltage. Systems such as PC add-in cards, which are part of a larger system over which the engineer usually has no control, must also consider external signals, impulses, and conditions. The most important of these is the system reset signal. This leaves most semiconductor reset control devices lacking in their ability to perform adequately in the add-in card environment.

Summit Microelectronics, Inc. has introduced a family of precision reset controllers with advanced features that provide a good fit to the PC add-in card environment. Moreover, these devices allow the user to improve reset control functionality while requiring fewer components! Summit's unique characteristic of incorporating E² trimmability in the reset circuitry allows the addition of industry standard E² memory to the device. The separate serial E²PROM can thus be eliminated from the circuit.

Implementing the SUMMIT Device
The incorporation of the Summit reset control device into the X2 modem design is quite simple. The X2 reference design published by Texas Instruments uses a Maxim 809 reset controller and a 93C66 Microwire serial E²PROM. The connections are shown in Figure 1.

 Figure 1: Existing Reset Scheme for X2 Chipset Processor

 

The Summit reset controller features a very convenient pinout for this situation. Since the reset control function requires only one active pin, the device was given the industry standard pinout of the 93C66 serial E²PROM. The only difference is pin 7, which, though unused, is tied high in the reference design (some 93C66 suppliers require a high input to disable test modes, with others is is a No Connect). With the Summit S93663 Precision Reset Controller, this pin is the RESET output. The user can now replace the 93C66 with the S93663 by changing one signal. Rather than tying pin 7 high, it is now tied to the Master Reset line (~MR) of the chipset processor. Since the S93663 now provides the reset function, the MAX809 can be eliminated, saving cost and board space (see Figure 2). The protocol of the S93663 is identical to the 93C66 so this becomes an "invisible" change to the processor.

 Figure 2: X2 System with Summit Device Providing Reset & Memory Functions

Additional Advantages: Reset Filtering
To this point, we have discussed the RESET pin of the Summit S93663 as an output. It is actually a bidirectional pin. The user can tie a reset input (from a manual reset button, for example) to the RESET pin and the S93663 will detect, filter, and act upon this input. This "reset filtering" capability gives the X2 modem designer increased reliability for free.

The system reset signal (RST) coming from the edge-connector is active high and is buffered on the board, usually by an inverter. Instead of using this buffered input as the card's reset signal, it is now used as an input to the Summit S93663. Upon receiving a reset signal from the system reset, the Summit device will assert the reset outputs and hold the reset state until a) the reset signal input is unasserted, or b) the t_PURST timeout period expires (130mS min), whichever period is longer. This is a critical aspect of the design. The pulse width of the incoming system reset signal varies greatly from machine to machine. Some PC platforms give the card slot a very short reset pulse, a pulse that is not long enough to ensure a reliable reset function for the chipset(s). In this case, the Summit precision reset controller is now acting as a reset signal conditioner, providing the chipset processor with an accurate and reliable reset pulse. This will eliminate issues related to platform variances of the reset signal and, more important, give the card designer control over this function.

The Summit device will still act as the supply voltage monitor, ensuring that the chipset is operating in a stable environment. When V<_CC drops below the threshold voltage, V_TRIP, for longer than 10nS, the Summit device will assert reset until V_CC stabilizes for a period no less than t_PURST. Summit Microelectronics, Inc. offers devices with a variety of V_TRIP settings and the unique "E²Analog" process allows for user-specified trip points.

Additional Advantages: Multiple Reset Requirements
Some designs may have multiple devices requiring reset, such as a PCMCIA card with both LAN NIC and modem capabilities. Both the Ethernet chipset and the X2 chipset require reset signals. The characteristics are not always identical. Some devices require active high reset signals while others require active low. The S93663 features complementary reset outputs that resolve this problem. When the Summit device receives the system reset pulse, both the reset line used as the input and the complementary reset line will be asserted and will stay asserted as described earlier. This scheme also eliminates the need to invert the incoming reset signal as an active high reset input will be connected to active high RESET pin (pin 7) of the Summit device and an active low reset input will be connected to the active low RESET (pin 6).

Figure 3 shows how the Summit device can be incorporated into a LAN NIC/modem design that incorporates multiple chipsets. The example shows the Ethernet controller, requiring an active low reset, and the X2 modem controller which requires an active high reset. Note that if it were convenient for the incoming system reset signal to be inverted, the connection would be made the RESET BAR I/O on pin 6 of the Summit S93663 device. Both the Ethernet controller and the X2 modem controller will be reset in the same manner. Any additional devices in the circuit, such as an I/O controller, speech CODEC processor device (for DSVD), Plug-and-Play controller, etc. that require reset inputs for reliable operation will have access to a reset signal of the desired polarity.

Figure 3: Design Requiring Multiple Resets of Opposite Polarities

Reliability Enhancement: Protection Against Data Corruption
The integration aspect of the Summit device offers additional benefits that are very important to the overall reliability and performance of the system. Stand-alone serial E²PROMs are inherently susceptible to data corruption, especially in brown-out (low V_CC) situations, power-up sequences, and power-down sequences. The S93663 features an internal lock-out feature that shuts down the write capabilities of the on-board memory when the system is in reset. This protects the E² from data corruption, doing so much more effectively than external write control pins. Data corruption can be, at best, a nuisance to the user, and at worst, catastrophic to unit operation. Elimination of even small incidents of data corruption can save considerable cost at all points of the product process.

Summary

Reliability Enhancements

• More Precise Reset Threshold
• Reset "Filtering" of External Reset
• Protection Against E² Data Corruption

Logistical Enhancements

• One Less Chip
• Less Board Space
• More Functions
• Lower Cost

The Summit precision reset controller family offers the designer of X2 modems a cost-effective method of increasing the performance of their design and increasing the reliability of operation. This will result in fewer production rejects and fewer product returns. If reset filtering is utilized, the Summit solution allows the designer a higher level of immunity from the wide variances in PC platforms. This allows for a reduced product evaluation and debug times which result in a faster time to market. The design-in is virtually invisible since the Summit part replaces an existing device which is 100% software compatible.

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