Onsemi CAT809STBI Microprocessor Supervisory Circuit: Features and Application Design Guide
In the realm of embedded systems and electronic design, ensuring a microprocessor operates reliably under all conditions, especially during power-up, power-down, and brownout scenarios, is paramount. The onsemi CAT809STBI is a dedicated microprocessor supervisory circuit engineered specifically to monitor power supply voltages and safeguard system integrity. This integrated circuit (IC) provides a critical function by generating a reset signal to the microcontroller (µC) or microprocessor (µP) whenever the supply voltage falls below a predefined threshold, preventing erratic code execution and system failure.
Key Features of the CAT809STBI
The CAT809STBI distinguishes itself through a combination of robust features tailored for high-reliability applications.
Precision Voltage Monitoring: The core function of the IC is to monitor the VCC supply voltage of the µP. It features a factory-trimmed reset threshold voltage, ensuring high accuracy without requiring external components. Common threshold options (e.g., 2.93V, 3.08V, 4.38V) cater to various logic levels.
Manual Reset Input: A pivotal feature is the inclusion of an active-low manual reset input (MR). This allows a physical button or an output from another logic device to force a system reset on demand, enhancing debugging capabilities and user control.
Assertion of Reset on Power-Up/Down: The device guarantees a proper reset signal during critical moments. It asserts a reset signal whenever VCC is below the threshold and maintains it for a minimum period (typically 140ms) after VCC rises above this threshold. This ensures the µP begins operation only after the power supply and system clock have stabilized.
Debounced Manual Reset: The MR input incorporates glitch suppression, providing switch debouncing to prevent unintended resets from contact noise, thereby increasing system robustness.
Low Power Consumption: Designed for always-on applications, the CAT809STBI operates with very low supply current, minimizing the power burden on the system, which is crucial for battery-powered devices.
Small Form Factor: Available in space-efficient packages like SOT-23-3 (for versions without MR) and SOT-23-5 (with MR), it is ideal for compact, portable electronics where board real estate is limited.
Application Design Guide
Integrating the CAT809STBI into a system is straightforward, but careful design is key to maximizing its benefits.
1. Basic Voltage Supervision:
The most common application is simple power monitoring. For a 3.3V system, a CAT809STBI with a 3.08V threshold is selected. The VCC pin is connected directly to the µP's power rail. The open-drain RESET output is connected to the µP's active-low reset pin, typically via a pull-up resistor to VCC if not internal to the µP. A decoupling capacitor (100nF) should be placed close to the CAT809STBI's VCC pin.
2. Utilizing the Manual Reset:
The MR pin can be connected to a tactile switch tied to ground. Pressing this switch pulls the MR pin low, forcing the RESET output active regardless of the VCC voltage. A series resistor (1-10kΩ) can be added between the switch and the MR pin to limit current.
3. Brownout Protection:
During a brownout—a temporary dip in the supply voltage—the CAT809STBI will immediately assert the reset signal once VCC drops below the threshold. It will hold the µP in reset until the voltage recovers and remains stable for the reset timeout period, preventing code corruption.
4. Power-On Reset (POR) Generation:
The internal timer of the CAT809STBI provides a fixed 140ms reset pulse upon power-up. This delay is often sufficient for system clocks (e.g., crystals) to stabilize before the µP begins executing instructions, which is a fundamental requirement for modern microprocessors.
5. Considerations for Noisy Environments:
In electrically noisy environments, ensure power traces are clean and short. The bypass capacitor on VCC is essential. For the MR line, if it is routed over long distances, consider adding a small capacitor to ground for additional noise immunity.
The onsemi CAT809STBI is an indispensable component for designers seeking to enhance the reliability and robustness of their microprocessor-based systems. Its integration simplifies design, reduces component count, and provides a fail-safe mechanism against power-related faults. By ensuring a controlled startup and shutdown sequence, it effectively prevents system lockups and data corruption, making it a superior choice for applications ranging from industrial controls and automotive systems to consumer electronics and medical devices.
Keywords: Microprocessor Supervisor, Power-on Reset (POR), Voltage Monitoring, Manual Reset, Brownout Protection.
