Protecting MCU digital input

Thread Starter

Willsy

Joined Aug 23, 2018
13
Good morning all.

I'd appreciate some comments/critique on a circuit to protect a digital input from both transients and over voltage. The circuit is presented below.

1712322962546.png

Explanation of Circuit (from right to left):
  1. /AlarmInput - input to the microcontroller (MCU) - active low (i.e. when low, an alarm condition is raised).
  2. R4 - weak pull up resistor to hold the input in the high condition.
  3. R5 & C5 - work together to provide a low-pass filter to filter out line ringing over long cable lengths. f is ~340Hz. The input changes state very rarely (measured in days).
  4. TVS1 - Transient suppression - Other inputs (using the same circuit conditioning as above) will eventually be added.
  5. R18 - Current limiting for TVS1 to protect it against (relatively) long duration line-ringing events where the voltage amplitude may trigger the TVS (see rationale below).

Observations:
I chose R5 (4.7K) to ensure a sufficiently low voltage is seen at the MCU when the input is shorted to ground via the connector on pins 3 & 4. I built the circuit, and can confirm that when the input is brought to ground, the voltage at the MCU input is 0.24 volts. Also R4 and R5 work together nicely to reduce current when the input is grounded (51uA).

The MCU (ATTINY441) datasheet states a voltage of 1.5 volts (in a 5V VCC configuration) to recognise a low voltage (if I'm reading it correctly), i.e. 5V x 0.3V = 1.5V so there should be no problems recognising a low voltage with this circuit, even on long cable runs.

1712323615128.png

Questions
My question mainly pertains to TVS1. I'm hoping that I can use it both as an ESD/transient suppressor, and also a voltage clamping device (therefore doing away with additional Schottky diodes) - but I'm not 100% sure that I am reading the datasheet correctly! My background is software engineering, so this hardware stuff is still somewhat confusing.

The datasheet says "The SP724 is a quad array of transient voltage clamping circuits designed to suppress ESD and other transient overvoltage events... The SP724 is connected to the sensitive input line and its associated power supply lines. Clamping action occurs during the transient pulse, turning on the diode and fast triggering SCR structures when the voltage on the input line exceeds one VBE threshold above the V+ supply (or one VBE threshold below the V- supply)."

The datasheet then presents a circuit providing over-clamp protection:
1712326675752.png
I interpret this as being able to provide protection for both high ESD voltages in the KV range, and low over/under voltage conditions such as may be observed when a line is 'ringing' when transitioning states. For ESD only protection, the current limiting resistor (R18) is (probably) not required due to the extremely small duration of an ESD event. However, during a line-ringing event when the circuit is taken to ground, these can last longer (microseconds/low milliseconds?) thus I added R18 to protect in this eventuality.

I'd be grateful for any opinions/comments either supporting or refuting my interpretation. Is there anyone here that has experience with this device? Can the circuit be simplified?

A link to the full datasheet is here: SP724AHTG datasheet.

Regards

Mark
 

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MrChips

Joined Oct 2, 2009
30,927
R4 100kΩ passive pullup seems a bit high to me. I would reduce that to 10k-33kΩ.
MCU GPIO pins usually have internal clamp diodes to both Vcc and GND. R5 provides current limiting.

Why did you draw the diagram with input from right to left?
It is customary to draw signal flow from left to right.
 

MisterBill2

Joined Jan 23, 2018
18,935
My take is that it is a good design with adequate precautions taken. If none of the other inputs on TVS1 are used, you could connect another one in parallel to increase the transient current handling ability.
But the arrangement looks good as it stands.
 

Thread Starter

Willsy

Joined Aug 23, 2018
13
My take is that it is a good design with adequate precautions taken. If none of the other inputs on TVS1 are used, you could connect another one in parallel to increase the transient current handling ability.
But the arrangement looks good as it stands.
Thanks very much for taking the time to comment. The suggestion regarding the unused inputs on the TVS is excellent. The device has two inputs, but the device has four inputs, so I've paralleled them as per your suggestion.

Thanks
 
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