Microcontroller getting heated up

[arunbhaskar]

I'm doing a project on heart rate monitor using finger sensor. I've enclosed the schematic below. The output of Schmitt Trigger i.e. the actual pulses goes to the INT0 pin of the microcontroller. I'm using a program to check the heart rate at the end of every 4th pulse. After calculating the BPM, I need to output it to a 16x2 LCD. I've enclosed the program below. I compiled it in Keil using Code ROM size:COMPACT in keil because my program didn't get built for Code ROM size small. I loaded the program onto the flash by using a serial port programmer.

I left the INT0 pin open just to check the program because according to my program, only welcome message would appear without any interrupt. When I switched the power supply ON, I didn't get any output on the LCD and found that the microcontroller was heated up. What could be the reason? Please help.

 

[IanP]

First, the circuit looks all right (see *), so are you sure it was assembled according to the diagram?
This microcontroller can be easily damaged by reverse polarity voltage, for example when Vcc is mixed up with GND, let’s hope this hasn’t happen ..
Second reason, still from the same category, is exposure of any pin to to high (Vcc+0.5V) or to low (Vdd-0.3V) voltages ..
(*) Looking at the component list, the 1N4942 diode that protects the /INT0 pin is not properly selected ..
Instead you should use a Shottky diode (Vf<0.3V) and if V[+] is higher than 5Vdc add a 4.7V Zener diode, too ..

If, while programing the microcontroller, everything went OK -- re-program it and try again, otherwise you’ll have to use new IC ..

IanP
:|

 

[the mask]

hi
maybe you draw current big current from control line as LCD line

so you can but resistance to decrease current and without drop voltage in LCD line

 

[arunbhaskar]

Thanks guys.
About IAnP's (*) remarks: I probably have uploaded the wrong schematic. I've used 4.7V zener in parallel with Schottkty diode like you've said in the PCB. Since I've done it on a PCB, the PCB making software(I used EAGLE) would make sure that I join the air-wires properly and there is very little chances of tracks getting short circuited. I've used a 40-pin base for mounting the uC. So, like you said I'll power the board up and check the voltage values of the required pins.
I have one more query. Do I have to add current limiting resistors on LCD lines, interrupt pin or any other pin for that matter? And yeah, I simulated the software on Proteus. I got the proper output with my program.
I've used a 12MHz crystal with AT89S51-24PC so that I get a exact delay of 1us for every tick of the timer. It should be ok right? I also heard that oscillator is a very delicate part and can conk off easily. How do I check if the oscillator is working.

 

[nandhu015]

It seems the problem lies with your hardware. As per the circuit everything seems to be correct.

Check for loose/wrong connections, dry soldering.

Nandhu