Test Measurement - Multi-Meters, Small Test Tools.

Wednesday, January 31, 2018

Digital Interface Third Part

These are notes made of Telegram Messenger - messages i sent. Only my Messages are combined here to create paragraphs. Related to Interfacing an Embedded System to external circuits.

Pull up means a resistor connected from a point to +V. Pull Down is Resistor connected from a Point to GND.  The point can be a Gate Input, Opamp Input or a Port Output. We use these when open collector output is used,  this way we can get a clear High Or Low or avoid a point to float(no potential)

Industrial Process Control Circuits

When uC is sleeping outputs should be written high. Which is also high impedance  like a insulator- The pull up of 100 k is put to keep the point high

Diode and Zener Clamp Protection

Here are two examples of IO protection from my older circuits. The diode or Schottky protection is for circuits measuring low voltage inputs like mV. Schottky forward drop on low currents is as low as 200mV. General Purpose diodes like 1N4148 have a drop of 400mV at low currents. Use low leakage zeners if you are protecting digital inputs. you can also diode clamp to VCC and GND or VDD too. Then inputs can swing betwwen VCC and VDD.

Diode and Zener Clamp Protection

When a low is written to the port a current Ib flows from vcc thru emmiter to base via a cuurent limit resistor. Beta times Ib which may be like 200 times ib flows from emitter to collector. This is the amplified Ic - Transistor switches on and saturates like a switch. - Vce may go as low as 0.5 volt on proper load

If Beta is 200. the base current is 3V/10k that is - 300 uA. X beta 200 gives. 60ma - 30ma per switch - Use Darlington to get more current they have beta of 2000 - Use TIP127 for many optical switches

Read this page  - Protecting ADC Inputs by Alan Walsh
Overdrive of ADC inputs generally occurs when the rails of the driving amplifier are significantly greater than the max input range of the ADC—for example, if the amplifier runs from ±15 V 
That Attenuator circuit looks ok, use a diode or zener clamp to protect the analog input port - see how much voltage it can take (not measure) absolute max ratings and clamp it. - Many zeners are leaky. Use good quality or try clamping with 1N4148.

Voltage above or below the max ratings can damage any pin of chip or device. Relative voltage. Most ports have short circuit protection but we should not take risks

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