GB2349757A

GB2349757A - A watchdog circuit for a mobile phone battery charging system

Info

Publication number: GB2349757A
Application number: GB9906365A
Authority: GB
Inventors: Anthony Paul Banks
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 1999-03-20
Filing date: 1999-03-20
Publication date: 2000-11-08
Anticipated expiration: 2019-03-20
Also published as: GB9906365D0; JP2000278108A; JP2000307285A; GB2349757B

Abstract

A watchdog circuit ensures that that the charging control circuit of a mobile phone is set to a safe state in the event of a fault occurring. An alternating output (S) from the CPU of the phone causes periodic discharge of a capacitor (C185) through a current sink (TR28-B) and maintains a low voltage on the emitter of a transistor (TR28A). With the low voltage maintained at its emitter the transistor remains switched on and charging continues. Loss of the alternating voltage (S) removes the periodic current sink from the capacitor (C185) which charges to a high voltage and by switching off the transistor (TR28A) terminates charging of the phone.

Description

Watchdog circuit This invention relates to watchdog circuits and in particular it relates to watchdog circuits operating a fail safe condition.

Mobile phones and many other portable devices rely for their operation on battery power from batteries which need regularly to be re-charged. When a battery in use with a mobile phone becomes discharged, the battery in use can be exchanged with a spare battery. Spare batteries can be maintained at a fully charged state. It is very often the case, however, that the battery in use with the mobile phone is recharged whilst remaining within the phone. Means for controlling the current supplied by an external charger to re-charge its battery are usually included in a mobile phone.

The circuitry for controlling the current supplied by an external charger is controlled by the CPU of the phone. A fault condition can occur which results in the phone turning on the charge current to the battery and then failing to turn it off when the battery becomes fully charged. Under these conditions the battery within the phone can become overcharged such that it swells in size and produces noxious fumes.

Watchdog circuits are known from the prior art and reference is made to WO 97/24652 assigned to Advanced Micro Devices Inc. and WO 98/43164 assigned to Robert Bosch GmbH for technological background.

It is an object of the present invention to provide a watchdog circuit for effective operation of a fail safe condition in which the circuit is composed of a minimum number of components.

According to the invention there is provided a watchdog circuit maintaining an output voltage level in a safe condition and altering said output voltage in response to a fault condition for control of an external circuit where the level of the output voltage depends upon the state of charge of a capacitor and in which, in a safe condition, periodic discharge of said capacitor through a current sink is effected by switching of said current sink by an alternating voltage.

The external circuit referred to here is external in the sense that it does not form part of the watchdog circuit but may be co-located with the watchdog circuit.

One example of the invention will now be described with reference to the figures in which like reference letters and numerals identify identical elements and wherein: figure 1 is a block diagram of the charging control circuit in a mobile phone, figure 2 is a circuit diagram of a watchdog circuit.

With reference to figure 1 a mobile phone is shown generally at 1 and an external charger unit is shown at 2. When charging is to proceed, a signal R from the microprocessor, CPU 6 is an input to the enable circuit 5. Whilst the signal R is present as an input to enable circuit 5, the switch 4 remains closed and charger 5 is connected to battery 3. A further signal S from CPU 6 is passed to the watchdog circuit 7. As long as the correct signal S is received by the watchdog circuit 7, then the watchdog circuit 7 has no effect on the operation of enable circuit 5 and charging continues.

With reference to figure 2, a circuit diagram is shown of the watchdog circuit 7 and the enable circuit 5 of figure 1. The output T connects to the gate of an FET which operates as a switch as indicated by block 4 of figure 1. The charging current to the battery is controlled by the FET of block 4. In order to turn the FET on and enable the charging current to flow into the battery, a high level voltage R is applied to terminal 111. The voltage input to terminal 111 is derived from an output port of the CPU 6. A high level on terminal 111 causes transistor TR28-A to turn on and the FET of block 4 to turn on also.

When the FET is on, the charge current is allowed to flow into the battery for charging to proceed.

The circuitry provided for the watchdog function comprises transistor TR28-B resistors R182, R155, R154 and capacitors C185, C184. With no input applied to terminal 112, TR28-B will remain switched off and will act as an open circuit. Whilst TR28-B is switched off the capacitor C185 will charge through R182 toward half of the voltage level R applied to terminal 111. The voltage R/2 will then be applied to the emitter of TR28-A, keeping TR28-A turned off and consequently keeping the FET of block 4 turned off.

An output S from the CPU6 will, under normal conditions, be applied to terminal 112. The output S is a 300KHz square wave, AC coupled via C 184 to the base of TR28-B. The square wave signal applied to the base of TR28 B periodically turns TR28-B on. Each time TR28-B is turned on it acts as a current sink and begins to discharge capacitor C185. The regular discharge of capacitor C185 maintains a low voltage across the capacitor and at the emitter of TR28-A, such that TR28-A remains on. With TR28-A switched on the FET of block 4 remains on and charging proceeds.

If a failure occurs and there is no output S from the CPU 6 to terminal 112 then C185 will charge up via R182, causing TR28-A and consequently the FET to turn off so that charging is stopped.

By this means the safety of the phone is improved over the prior art and in the event of software crash or a hardware problem, the phone cannot continue to overcharge of a battery.

As the signal from the CPU 6 is AC coupled via C184, an alternating signal is needed for the watchdog circuit to maintain a satisfactory output to the enabling circuit. An additional safety feature is therefore provided as the processor CPU 6 has to be actively running code to generate the required alternating waveform. If the processor has stopped due to a broken connector in the supply to the clock for example, the charger cannot be left turned on. The circuit may be used for other purposes, such as to generate a system reset in the event of a software crash, or for monitoring a safety critical control circuit to ensure setting to a safe state in the event of a software error.

Claims

Claims 1. A watchdog circuit maintaining an output voltage level in a safe condition and altering said output voltage in response to a fault condition for control of an external circuit where the level of the output voltage depends upon the state of charge of a capacitor and in which, in a safe condition, periodic discharge of said capacitor through a current sink is effected by switching of said current sink by an alternating voltage.
2. A watchdog circuit as in claim 1 in which the output voltage is maintained at a low level in a safe condition and altered to a high level in a fault condition.
3. A watchdog circuit as in claim 1 or 2 in which the external circuit controlled by the output of the watchdog circuit is a charging control circuit within a mobile phone.
4. A watchdog circuit as in claims 1 to 3 in which the alternating voltage switching the current sink is a square wave output from a microprocessor.
5. A watchdog circuit as in any preceding claim in which the current sink is a transistor.