31-03-2012, 12:35 PM
Interfacing ADC 0808 with 8051
Interfacing ADC 0808 with 8051.doc (Size: 191 KB / Downloads: 66)
In most of the digital system micro controller needs to take input from Analog sensor or transducer. Analog signal are most common input signal for embedded system. Sensor or transducer Such as Temperature, Motion, pressure, Humidity are analog. You will need to design a circuit to program AT89c51 or 8051 for converting analog input to equivalent digital data.
Reference Voltage or Step Size
Step Size or Reference voltage can be calculated from equation Step Size = (VREF(+) + VREF(-))/2 If we consider VREF(-) = Gnd and VREF(+) = 5V then step size = 5/256 = 19.53 mV. If you using temperature sensor LM35 then step size is 10 mV. 10 mV step size can be set by VREF(-) = Gnd and VREF(+) = 2.56V.
ALE
ALE (Address Latch Enable) is L - to H pulse to latch in address.
START
ADC starts conversation on receiving this signal high to low. Successive approximation register (SAR) is reset on the positive edge of the start conversion (SC) pulse. The conversion is begun on the falling edge of the start conversion pulse.
EOC (End Of Conversion)
This is an active low O/P signal from ADC on receiving low EOC micro controller Come to know that the converted data is ready to be picked up.
OE (Output Enable)
ADC converts the analog input to its binary equivalent and holds it in an internal register. L- to-H OE is used to enabling TRI - STATE output, so Converted data can be red from data pin D0 – D7.
Clock signal
Clock signal cannot be faster then 100ms.It can be external or provided from microcontroller crystal using 4 D- F/F as frequency divider. Single D- F/F divides frequency by 2.
Signal conditioning
If sensor signal is very low then apply proper amplification. You need simple opam amplifier. Normally sensor O/P is in range of mV and affected by noise very easily for better accuracy apply 2 - 5 gain. If you using temperature sensor LM35 its O/P is 10mv/C and it is advised to amplification by gain of 2 – 5.