Abstract:
A programmable low voltage reset apparatus for a device having a plurality of power supplies comprises a low voltage signal generator for sensing when a power supply output decreases below a predetermined voltage and generating a reset signal, a reset selector for selecting one of the power supplies, and a programmable reference voltage for varying a reference voltage according to the voltage of the selected power supply.

Description:
RELATED APPLICATION DATA 
   This application claims priority from U.S. Provisional Application No. 60/556,179 filed Mar. 24, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to electronic circuits and, more particularly, to circuits for performing a reset scheme in devices having multiple power supplies. 
   2. Description of the Related Art 
   Low voltage reset schemes are used in devices required to perform operations from the time a low voltage indication is generated to the time power is lost for all operational purposes. These operations may include protecting values in some important status registers by storing them to a non-volatile storage on board or device, protecting a data being written to a non-volatile memory, and other related functions. These low voltage reset schemes are generally used in microcontroller-based circuits. In one conventional embodiment, certain 5V microcontroller devices have used these kinds of circuit to protect on-chip flash memory when power falls below 4.1V. 
     FIG. 1  is a block diagram of a conventional low voltage inhibits reset signal generation circuit  1 . As shown, the conventional low voltage reset scheme is based on comparing a resistor divided value of the power supply (Vdd) with a stable reference like a bandgap voltage (Vbg). The low voltage reset signal LVI_reset is coupled to a reset logic for issuing a “reset” signal to a pin depending on the voltage level of the supply voltage (Vdd). 
   The conventional solution in  FIG. 1  comprises a voltage reference generated by a resistor divider network  2 , where at the top of the network is a power supply (Vdd) and the output of the resistor network comprises a voltage reference signal  3 . The voltage reference signal  3  is coupled to the negative input of a comparator  5 . The positive input of the comparator  5  is coupled to a bandgap reference voltage Vbg, and the output of the comparator  5  forms a low voltage indicator reset signal LVI_reset. 
   A disadvantage of the conventional solution is that the low voltage inhibit reset signal generation circuit  1  cannot support devices having multiple power supply levels (multiple Vdds). When there are multiple Vdds on a chip, a reset circuit needs to be activated when one or more of the Vdds is lost. It is also desirable to have a user select whether to activate reset on the device when one specific Vdd is lost or when any one of the Vdds is lost. 
   It would be desirable to have a device capable of supporting multiple power supply levels because the device may need to interface with other devices operating under different power supply levels on a board. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the low voltage reset apparatus for a device having a plurality of power supplies comprises a low voltage signal generator for sensing when a power supply output decreases below a predetermined voltage and generating a reset signal, a reset selector coupled to the low voltage signal generator for selecting one of the power supplies, and a programmable reference voltage generator for varying a reference voltage according to the voltage of the selected power supply. 
   In one embodiment of the low voltage reset apparatus, the reset selector comprises a first state and a second state. 
   In one embodiment, the reset selector is set to the first state during a power up operation, and the low voltage signal generator is disabled at the first state. 
   In one embodiment of the low voltage reset apparatus, the reset selector is set to the second state after the power up operation, and the low voltage signal generator is enabled at the second state. 
   In one embodiment, the reset selector is controlled by firmware. In one embodiment, the reset selector comprises a latch element coupled to a multiplexer. 
   In one embodiment of the low voltage reset apparatus, the reference voltage generator is controlled by firmware. In one embodiment, the programmable reference voltage generator comprises a resistor ladder. 
   In one embodiment of the low voltage reset apparatus, the low voltage signal generator is disabled during a power up operation if the selected power supply is at a first voltage and enabled after the power up operation if the selected power supply is at a second voltage higher than the first voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of embodiments of the invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings. 
       FIG. 1  is a block diagram of a conventional low voltage inhibit reset signal generation circuit. 
       FIG. 2  is a programmable low voltage inhibit reset signal generation circuit for multiple power supply applications. 
       FIG. 3  is a low voltage inhibit reset signal generation circuit having a programmable reference voltage. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The invention will be described below with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     FIG. 2  shows a programmable low voltage inhibit reset signal generation circuit  10  for multiple power supply applications. In one embodiment of a programmable low voltage induced reset scheme, a device  90  such as for example, a 3.3V microcontroller, may operate with a plurality of power supply devices. On the 3.3V microcontroller device, there may be two power supplies V 1  and V 2 , and the device performs a reset operation when any of the power supplies is lost. 
   The programmable low voltage inhibit reset signal generation circuit  10  comprises a multiplexer  20  having first  22  and second  24  inputs, a control input  26 , and an output  28 . This circuit is part of reset block inside device  90 . The first input  22  of the multiplexer  20  is coupled to a power supply ground level Vgnd. The second input  24  of the multiplexer  20  is coupled to a first supply voltage low voltage inhibit signal LVI_ 1 . The low voltage inhibit signal LVI_ 1  is low if the voltage at the first supply V 1  is above a predetermined threshold, and high if the voltage at the first supply V 1  is below the predetermined threshold. If a requirement arises to add a third power supply to a device, the first input  22  can be connected to a low voltage inhibit signal from a third power supply provided that the signal is generated similar to the low voltage inhibit signal LVI_ 1 . 
   The programmable low voltage inhibit reset signal generation circuit  10  further comprises a flip-flop  30  having an output coupled to the control input  26  of the multiplexer  20 . The flip-flop  30  has an enable input  32 , which is coupled to a power on reset (POR) signal. The flip-flop  30  further comprises a data input  34  and a clock input  36 . 
   The programmable low voltage inhibit reset signal generation circuit  10  further comprises a NOR gate  40 . A first input  42  to the NOR gate  40  is coupled to a second supply voltage low voltage inhibit signal LVI_ 2 . The second supply voltage low voltage inhibit signal LVI_ 2  is low if the voltage at the second supply V 2  is above a predetermined threshold, and high if the voltage at the second supply V 2  is below the predetermined threshold. 
   The output  44  of the NOR gate  40  comprises a low-voltage inhibit reset signal LVI_reset, which is a function of the first supply voltage low voltage inhibit signal LVI_ 1  and the second supply voltage low voltage inhibit signal LVI_ 2 . The programmable low voltage inhibit reset signal generation circuit  10  may further comprise an inverter  50  after the NOR gate  40 , depending on the requirements of a particular application. 
   In one embodiment, the first power supply V 1  may be taken to multiple power supply levels (for example, Vdd 1  at 3.3V or Vdd 2  at 5V) based on user application and the second supply V 2  is at Vdd 1  (3.3V) only. Since the voltage level at the first power supply V 1  depends on the user application, the voltage at which low voltage reset need to be activated may also be decided by the user. For example, if the first power supply V 1  is at 5V, then a low voltage reset may be activated when that supply falls below 4.1V. If the first power supply V 1  is at 3.3V, both power supplies V 1  and V 2  may be derived from the same source, and the low voltage activated reset may be performed by the circuit on the second supply V 2 . 
   In one exemplary embodiment, the voltage at the first supply V 1  is at approximately 5V and the threshold for the first supply voltage low voltage inhibit signal LVI_ 1  is approximately 4.1V. In one exemplary embodiment, the voltage at the second supply V 2  is at approximately 3.3V and the threshold for the second supply voltage low voltage inhibit signal LVI_ 2  is approximately 2.85 Volts. 
   In one embodiment, the first supply V 1  may be 3.3V or 5V depending on user application. That is, the voltage at V 1  is not determined until the user makes a selection well after power-up of the device  90 . Hence, a low voltage reset signal LVI_ 1  from the first power supply V 1  cannot be used to activate the reset cycle during power-up since the device will always remain in reset if its application is to power up only to 3.3V. When the voltage at the first supply V 1  is at 3.3V, the first supply voltage low voltage inhibit signal LVI_ 1  will always remain high; hence, a user program will not use the first supply voltage low voltage inhibit signal LVI_ 1  for chip reset. 
   However, a first supply voltage induced reset is still required during a power loss. A user may write into a configuration register through firmware whether to activate a low voltage reset signal LVI_L during a power loss on the first power supply V 1 . In one embodiment as shown in  FIG. 2 , the configuration may be implemented as a D-type flip flop (DFF)  30 . If the first and second power supplies V 1  and V 2  are both 3.3V, then the user does not write anything to the register to change its state, and reset may be activated through the second power supply V 2  only. Typically, when both power supplies are at a common level, they are normally generated from one source on board. 
   If the user decides to use the first supply voltage low voltage inhibit signal LVI_ 1  information by writing into a configuration register, the first supply voltage low voltage inhibit (LVI) signal activated reset cycle will start once that supply falls below the first threshold voltage (4.1 V in one embodiment). This is applicable when the user application for the first power supply V 1  is at voltage level Vdd 2  (5V in one embodiment) and generated from a different and independent source on the board. 
   Thus, the programmable low voltage inhibit reset signal generation circuit  10  can be designed such that a user determines the conditions at which a low voltage induced reset signal is activated. Configuration registers which can be written through firmware after power-up can be provided and, thus, whether to use the reset through any of the power supplies upon power-up may be programmed. If a single power supply pin on a device can be taken to multiple supply levels based on the application, a configuration register may be associated with that power supply. The configuration register will be initialized to a logic state (upon power-on), which will disable the dependence of reset activated through that power supply upon power-up. This will ensure that the device will power up (not remain in reset) even if the power supply is assigned to the lowest possible value as per the user&#39;s application. If the power supply application on that pin is different, then user can write the configuration register and decide whether to use the low voltage reset from that supply during power loss. 
     FIG. 3  shows an embodiment of a programmable low voltage inhibit reset signal generation circuit  10  having a programmable reference voltage Vbg_prg. Usually a fixed bandgap reference voltage Vbg (1.2V) is used as a reference. The fixed bandgap reference voltage Vbg can be amplified by a user by writing into a register  70  through firmware if the Vdd is higher. Tapping points  65  in resistor ladder  60  may be controlled by configuration bits in a register  70  programmed by a user. If a user programs the configuration bits in a register  70  to tap at a lower end of a resistor ladder  60 , then programmable reference voltage Vbg_prg will be set to a higher voltage (for example, 1.8V), which is suitable for a Vdd 2  of 5V. If a user programs the configuration bits to tap at a higher end of resistor ladder  60 , then programmable reference voltage Vbg_prg will be set to a lower voltage (for example, 1.2V), which is suitable for a Vdd 2  of 3.3V. 
   In one embodiment, the programmable reference voltage Vbg_prg may be set to be suitable for a Vdd 2  of 3.3V and the threshold for the first supply voltage low voltage inhibit signal LVI_ 1  is approximately 2.85V. That is, the first supply voltage low voltage inhibit signal LVI_ 1  is low if Vdd 2  is above 2.85V and high if Vdd 2  is below 2.85V. In another embodiment, the programmable reference voltage Vbg_prg may be set to be suitable for a Vdd 2  of 5V and the threshold for the first supply voltage low voltage inhibit signal LVI_ 1  is approximately 4.1V. 
   In one embodiment, the programmable reference voltage Vbg_prg may be set to be suitable for a Vdd 2  of 3.3V and the threshold for the second supply voltage low voltage inhibit signal LVI_ 2  is approximately 2.85V. 
   A programmable low voltage inhibit reset scheme for multiple power supply operation has an advantage that includes protecting external devices (flash) from getting corrupted when any one of the power supplies is lost. In the event that power is lost when a device is performing a flash (external or internal) write operation or if the voltage level falls below the operating voltage of flash before the write operation is complete, then the write operation will not be successful and existing data in flash may get corrupted. This may result in loss of data stored in the non-volatile memory. It is important to detect power supply droop below a pre-determined threshold and complete the current flash write operation before the supply droops below the minimum operational voltage for flash. 
   A further advantage includes that the programmable low voltage inhibit reset scheme may avoid network jams in the event of power failure on any of the nodes (devices) connected in the network. In the case of a network operation, when the power supply of the device droops below a certain threshold, we need to activate a reset, communicate it to interfacing devices and stop driving the network so that the other nodes or devices on the same network will not be affected and communication will not be disturbed. 
   Moreover, the programmable low voltage inhibit reset scheme of the disclosed invention does not need any external low voltage detect circuits even though the devices operate in a multiple power supply environment. 
   It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention. 
   Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. 
   Having described exemplary embodiments of the invention, it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. Therefore, it is to be understood that changes may be made to embodiments of the invention disclosed that are nevertheless still within the scope and the spirit of the invention as defined by the appended claims.