8051 hardware problem

[d@nny]

danny price of 89s51 and 89c51 or 89s52 is round about under 130 pkr...tushki you mean that i use logic level zero to activate my circuit conected at port instead of using logic level 1...and i coudnt understand the hardware inverter will you kindly post the shematics of hardware invertor...thnks

Where r u from?
The one soloution for your problem is that use pnp transistor for this problem. but becareful as if u tried to on a 12volt relay with pnp transistor the 12 volt will flow towards the base of the transistor your transistor is at 5 volt so this 12volt can flow in your mcu through base and your mcu will burn so you can use pnp for 5 volts only. the other and the most simplest solution for driving 12 or 30volt relays is to attach another transistor with for driving 12volt or 30volt relay or something. The complete solution for your problem is ******** UDN2982 ********* . It is easily available.

---------- Post added at 18:50 ---------- Previous post was at 18:46 ----------

Recommended for high-side switching applications that benefit from
separate logic and load grounds, these devices encompass load supply
voltages to 80 V and output currents to -500 mA. These 8-channel source
drivers are useful for interfacing between low-level logic and high-current
loads. Typical loads include relays, solenoids, lamps, stepper and/or servo
motors, print hammers, and LEDs.
All devices may be used with 5 V logic systems — TTL, Schottky TTL,
DTL, and 5 V CMOS. The UDN2981A, UDN2982A, UDN2982LW, and
A2982SLW are electrically interchangeable, will withstand a maximum
output off voltage of 50 V, and operate to a minimum of 5 V; the UDN2983A,
UDN2984A, UDN2984LW, and A2984SLW drivers are electrically interchangeable, will withstand an output voltage of 80 V, and operate to a
minimum of 35 V. All devices in this series integrate input current limiting
resistors and output transient suppression diodes, and are activated by an
active high input.

Just check datasheet of UDN2982.

 

[ALERTLINKS]

That's by design of x51 processor and can't be avoided. All port pins, that have an internal pull-up, e.g. P1 will go to high state during power-up and reset. Your external circuit must be designed to deal with this behaviour. In some cases, you may need to invert a signal. Basically, there's no problem if all signals are defined active low.

Its fact but often people dont design that way. We used to tackle this problem by delaying power supply to external circuits. Search for switch on delay circuits. Or by asighning a mcu pin providing a small delay, output goes to 0 which in turn switches power rail to relays etc through transistor switching. At first mcu pins are high by default until set to output 0 in software. This delay causes problem .

 

[shamikrudra]

Use inverter like 7414 in between your relay and controller. this will solve your problem very easily. and your controller will be safer.

 

[aliraza786]

thnku ALL everyone has awsome solution of this problem it helped me alot and i am from Lahore pakistan........

 

[romel_emperado]

as FVM suggested you can invert your signal.. you can do it in your code or in the hardware..

at power on state the port pins will go high and if you want to avoid that use logic low or 0 to trigger your external circuitry and logic high or 1 to disable something..