The ADC0809 is an 8-channel, 8-bit successive approximation A/D analog-to-digital converter manufactured by National Semiconductor. There is an 8-channel multiswitch inside, which can latch the decoded signal according to the address code, and only gate one of the 8 analog input signals for A/D conversion. It has been phased out at present.
Main features
1) 8 input channels, 8-bit A/D converter, that is, the resolution is 8 bits.
2) It has a conversion start-stop control end.
3) Conversion time 100μs (when the clock is 640kHz), 130μs (when the clock is 500kHz)
4) Single +5V power supply
5) Analog input voltage range 0~+5V, no zero point and full scale calibration required.
6) The working temperature range is -40~+85 degrees Celsius
7) Low power consumption, about 15mW.
Internal structure
The ADC0809 is a CMOS monolithic successive approximation A/D converter with an internal structure as shown in the figure, which consists of 8 analog switches, address latches and decoders, comparators, 8-bit switch tree A/D converters, successive approximation registers, logic control, and timing circuitry.
External Features (Pin Function)
The ADC0809 chip has 28 pins in a dual in-line package, as shown. The functions of each pin are described below.
IN0~IN7: 8 analog inputs.
2-1~2-8: 8-bit digital output.
ADDA, ADDB, ADDC: 3-bit address input line for strogging one of 8 analog inputs
ALE: Address latch allows signal, input, active high.
START: A/D conversion starts the input pulse and enters a positive pulse (at least 100ns wide) to start it (the rising edge of the pulse resets 0809 and the falling edge starts the A/D conversion).
EOC: The end of A/D conversion, output, when the A/D conversion ends, this terminal outputs a high level (all the way low during conversion).
OE: Data output allows signal, input, active high. When the A/D conversion is complete, a high level is entered at this end to open the output tri-state gate and output the digital quantity.
CLK: Clock pulse input. THE CLOCK FREQUENCY IS REQUIRED TO BE NO HIGHER THAN 640KHZ.
REF(+), REF(-): Reference voltage.
Vcc: Power supply, single +5V.
GND: Earth.
Editing this section of the work process first enters the 3-bit address, makes ALE=1, and stores the address in the address latch. This address is decoded and gated to one of the eight analog inputs to the comparator. The START rising edge resets the successive approximation registers. The falling edge initiates the A/D conversion, after which the EOC output signal goes low, indicating that the conversion is in progress. Until the A/D conversion is complete, the EOC goes high, indicating that the A/D conversion is complete, and the resulting data has been stored in latch, and this signal can be used as an interrupt request. When the OE input is high, the output tristate gate is opened and the digital output of the conversion result is output to the data bus.
Transmission of conversion data The data obtained after A/D conversion should be transmitted to the single-chip microcomputer for processing in time. The key issue in data transfer is how to confirm the completion of the A/D conversion, because only after the confirmation is completed, the transfer can take place. There are three ways to do this.
(1) Timed transmission mode
For an A/D converter, the conversion time is known and fixed as a technical indicator. For example, the ADC0809 conversion time is 128μs, which is equivalent to 6MHz MCS-51 microcontroller for a total of 64 machine cycles. According to this, a delay subroutine can be designed, and this subroutine is called after the A/D conversion is started, and when the delay time is up, the conversion must have been completed, and then the data can be transmitted.
(2) Inquiry method
The A/D conversion chip consists of a status signal indicating that the conversion is complete, such as the EOC side of the ADC0809. Therefore, the status of EOC can be tested in query mode to confirm whether the conversion is complete and then data transfer.
(3) Interruption mode
The status signal (EOC) indicating that the conversion is complete is used as an interrupt request signal, and the data is transmitted in an interrupt manner.
Regardless of which method is used, once the conversion is confirmed, the data can be transmitted via instructions. When the exit address is sent first and the signal is valid, the OE signal is valid, and the conversion data is sent to the data bus for the single-chip microcomputer to accept.
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