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The ADC0809 is a CMOS-based 8-channel, 8-bit successive approximation analog-to-digital converter (ADC) developed by National Semiconductor. It integrates an internal 8-channel multiplexer that allows the device to select and latch one of the eight analog input signals based on the address code provided. This makes it ideal for use in microcontroller-based systems where multiple analog inputs need to be converted into digital data.
In terms of input requirements, the ADC0809 accepts a unipolar analog signal with a voltage range between 0V and 5V. If the input signal is too weak, it must be amplified before being fed into the ADC. Additionally, the analog input should remain stable during the conversion process. If the input changes rapidly, a sample-and-hold circuit should be used before the signal reaches the ADC to ensure accurate results.
The ADC0809 has several control lines that manage its operation. The ALE (Address Latch Enable) line is used to latch the channel address provided through the A, B, and C pins. These three address lines determine which of the eight analog channels (IN0 to IN7) will be selected for conversion. The channel selection table helps users identify which combination of A, B, and C corresponds to each input channel.
For the digital output and control lines, there are 11 signal lines involved. The ST (Start) signal initiates the conversion process. When the ST signal transitions from low to high, it triggers the start of the conversion. During this time, the EOC (End of Conversion) signal indicates whether the conversion is still in progress or has completed. Once the conversion is done, the EOC goes high, signaling the system to read the data.
The OE (Output Enable) signal controls the data output from the ADC’s internal latches. When OE is high, the converted digital data is available on the D7-D0 output lines. When OE is low, the output lines go into a high-impedance state, preventing interference with other devices.
The ADC0809 requires an external clock signal, typically at a frequency of 500kHz, as it does not have an internal clock circuit. The reference voltages VREF(+) and VREF(-) define the full-scale range of the ADC, ensuring accurate conversion of the analog signal into a digital value.
In application design, the ADC0809 can be directly connected to microcontrollers like the AT89S51 due to its built-in output latches. To initialize the ADC, both the ST and OE signals should be set to low. Then, the desired channel address is sent to the A, B, and C pins. A positive pulse of at least 100ns is applied to the ST pin to start the conversion. The system then monitors the EOC signal to determine when the conversion is complete. Once EOC is high, the OE signal is set to high, allowing the microcontroller to read the converted data.
The ADC0809 operates in a pulse-start mode. After the ST signal is triggered, the EOC signal becomes active after approximately 10 microseconds. This means the system must wait for the EOC signal to go high before reading the data. Programmers can either use a polling method or an interrupt-driven approach to detect the EOC signal and retrieve the converted data. Alternatively, a delay can be implemented after starting the ADC to allow sufficient time for the conversion to complete.
A typical program flowchart for using the ADC0809 includes steps such as initializing the ADC, selecting the appropriate channel, starting the conversion, waiting for the end of conversion, and finally reading the digital output. This ensures a systematic and reliable way to interface the ADC with a microcontroller.
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