Method and circuit for detecting a brown out condition

A brown out detection circuit includes a sense amplifier for sensing a current level exhibited by a flash cell. Combinatorial logic is coupled to the sense amplifier for identifying a program condition of the flash cell based on the sensed current level, including a brown out condition, in order to provide a warning to avoid potential malfunction from a brown out condition.

FIELD OF THE INVENTION

The present invention relates to brown out detection systems.

BACKGROUND OF THE INVENTION

Recent advances in technology have led to an increase in performance and a decrease in size of semiconductor microcontroller or microprocessor chips. Microcontrollers generally receive power from a supply voltage that is external to the microcontroller. In order to ensure proper operation, many conventional microcontrollers employ a brown out detection system.

Brown out detection systems typically detect when a supply voltage level drops below a minimum level that is necessary for proper operation of the microcontroller and/or the item using the microcontroller. Brown out detection systems help protect the microcontroller against total power failure and against “dips” in the received voltage signal.

The ability to help protect a microcontroller and maintain proper functioning is widely beneficial, but of particular importance in critical application environments, including medical environments. A need exists for a brown out detection system that is straightforward, efficient, and effective. The present invention addresses such a need.

BRIEF SUMMARY OF THE INVENTION

Aspects of brown out detection are described. Included in these aspects is a brown out detection circuit. The brown out detection circuit includes a sense amplifier for sensing a current level exhibited by a flash cell. Combinatorial logic is coupled to the sense amplifier for identifying a program condition of the flash cell based on the sensed current level, including a brown out condition, in order to provide a warning to avoid potential malfunction from a brown out condition.

Through the present invention, the program state of flash memory is employed in providing brown out detection. In a straightforward and efficient manner, the present invention provides detector circuitry that monitors for a drop in a current level of a programmed flash memory cell in order to identify a brown out condition. Potential malfunction can be avoided by use of the warning signal to allow for reprogramming or shut down of a controller. These and other advantages of the present invention will be more fully understood in conjunction with the following detailed description and accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is related to brown out detection systems. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.

In accordance with the present invention, the program state of flash memory is employed in providing brown out detection. As is well understood in the art, flash memory is a type of EEPROM that can be erased and reprogrammed in blocks. Many modem PCs have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. A typical flash memory comprises a memory array which includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.

In accordance with the present invention, the data in a flash cell is utilized in providing brown out detection.FIG. 1illustrates a circuit diagram of a transimpedance sense amplifier to demonstrate providing brown out detection with a flash cell in accordance with the present invention. As shown, the circuit includes an amplifier100comprising transistors102and104. Coupled to the amplifier100is a current comparator106comprising transistors108and110. Also coupled to the amplifier is a flash cell represented by a current source112. The data from the current comparator106is sampled and held by a latch114. Inverter116buffers the output from the latch114to provide a DATA_OUT signal.

A second current comparator118, comprising transistors120and122, is coupled to the amplifier100. A latch124is coupled to comparator118. Combinatorial logic126in the form of an XOR gate is coupled to the latch124and outputs a signal (WARNING) as a brown out detection output.

In operation, when the current of current source112is low for an unprogrammed cell, the DATA_OUT will be low. When the current is high for a programmed cell, the DATA_OUT will be high. In detecting a brown out condition, the maintenance of the charge on a flash cell is monitored by monitoring the current seen. When a flash cell starts to lose its charge, the current will drop. If the current drops below a predetermined threshold (e.g., if the current drops below about 80% of a maximum value), the WARNING signal is output based on the combination through XOR gate126of the sampled states of current comparators106and118latched by latches114and124.

In this manner, the brown out detector senses when the flash cells start to lose their contents. This may happen, for example, if the flash memory is subjected to high temperature over an extended period of time, subjected to radiation, etc. However, it will also detect when the cell current is too low, which will happen at low voltage supplies, or even when the clock frequency of the chip is too high (and the flash memory has not got enough time to resolve correct data). Thus, for critical applications (such as medical, for example), the warning signal can be used to prompt a controller to reprogram itself or completely shutdown to prevent malfunction, as is well appreciated by those skilled in the art.

FIG. 2illustrates a signal diagram from a simulation of operation of the circuit ofFIG. 1. It should be appreciated that this simulation reflects a loss of charge in the cell. Similar simulations give a brown out indication if the supply voltage drops or the clock frequency is too high.

Referring toFIG. 2, signal line200represents current levels of a flash cell (i(ieecell)). Signal line202represents voltage levels of the sample signal (v(sample)). The data output from latch114and buffered through inverter116is represented as voltage levels by signal line204(v(data_out)). Signal line206represents voltage levels of the warning signal (v(warning)) output from combinatorial logic126. Three program conditions of the flash cell are represented in the signal lines. Namely, an unprogrammed cell condition is present at 0.5 us (microseconds), a good programmed flash cell is present at 2.5 us, and a slightly discharged programmed cell (brown out) condition is present at 4.5 us. As shown, the warning signal goes high in correspondence with the drop in current from the flash cell, indicative of a loss of charge in the cell and thus a drop in the supply voltage.