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Serial Interface 7135 to 8751 Reads Analog Values


Using a ICL7135 A to D Convertor with a Microcontroller is easy with the Serial Interface. The advantage is you could use an Opto Coupler to protect your Embedded System.

uC Input and Output I/O is Keyboard and LCD when you build a Calculator or a Text/Morse Communicator. When you control a motor it is Drive/Speed and Direction for Motor and Tacho or Opto Counters for Feedback. In both cases you have Input and Output which is Logic or Analog which has to reach the CPU.

Atmel-AT89S52-Microcontroller

Getting the analog into the uC is done here with ICL7135, with a simple two wire serial interface. Here a Pair of CD4538 MMV is used to shape the pulse just right for uC data transfer,

ICL7135 Intersil, 8751-8031-8951, the circuit for uC chips are not drawn in the circuit below as they are in other circuits. See datasheets or application notes too.

The Busy signal and clock when anded (AND) will give a burst of pulses. Count them and deduct 10,001 from the virtual uC counter. This is the reading. See ICL7135 at Maxim IC.


CD4538 Monostable
                        Pulse Shaping Circuit

PNG-Schematic  -   PDF-Circuit   -  Source
Serial Interface 7135 to 8751 Reads Analog Values - del20001
To view circuit above, Click the Link of PNG or PDF, PNG can be Drag Scrolled with Mouse


U2B sends a short interrupt pulse to uC which means 'busy' of 7135 has gone from high to low and reading in counter of uC is valid, also reset counter to 0 for new reading next time 'busy' goes high. count the clock and substract 10,001 from that, you got a reading.

BUSY 'AND' CLK '-' 10,001 '=' READING

ICL7135 Pin 11 +5V, Pin 1 -5V, Pin 24 digital GND

SDCC - Small Device C Compiler

The input of ICL7135 has to be protected from Overvoltage both negative and positive. This is done by a Clamping circuit made of two back to back low leakage Zeners. Some zeners may leak more, see the datasheet. Use Ordinary Diode clamping circuit with 2 nos 1N4148 in series to the + and - rails. Change ICL7135 to 200mV FS for Diode Clamping.

Zener Diode Voltage Clamping Protection

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