CH32V006 input - output impedance

[boda-akos]

This data is not really present in the PDF-s.

Analog Inputs have an approximate 75 kOhm impedance, considering that the ADC resolution is 12 bits (0-4095) this load will cause 1 count ADC error already if the driving source's internal impedance is 18 Ohm !! (75000 / 4096). Drive the analog inputs always from an opamp voltage follower to ensure low impedance.

Method to measure : 3.3V input on ADC0 channel, resistor in series with the input, increase resistor until the ADC value drops to half scale (2047).

Inputs like pinMode( pin, INPUT ) are very high impedance. No worries.

Output like pinMode (pin, OUTPUT) source about 40mA and sink 46mA. Internal impedance 32.5 Ohm sourcing and 22 ohm when sinking. Method to measure : 50 Ohm load on PC0 output switched to 3.3V VCC or 0V VSS. Blink example on output runs at 1 kHz. Voltage drop measured with Rigol DS1054 oscilloscope. Sourcing pulse p-p drops to 2.0V. Sinking pulse p-p drops to 2.3V. Other output pins have very similar characteristics according to data sheet (variations 50mV, page 33 of CH32..DSO.pdf)

Conclusion : LED drive is possible without current limiting resistors, a 4-digit 7 segment LED multiplexed 1/4 of the time is driven by 10mA average current which is ideal.

[maxint-rd]

Hello @boda-akos , that's interesting information. My knowledge about general electronics (and particularly about impedance) falls way too short to be able to appreciate this at all levels, but I do like reading your conclusion.

Conclusion : LED drive is possible without current limiting resistors

In the past months I scavenged several vape-sticks (found on the side of the road), that feature a dual 2-digit 7-segment LED display (and also include a USB-C charger). Those displays use Charlie-plexing to minimize the pin-out and I was wondering how I could best reuse them. Your conclusion gives me the courage to try direct connections without to much risk of damaging anything. Thanks!

Drive the analog inputs always from an opamp voltage follower to ensure low impedance.

Perhaps a noob-question: do you mean one should always use the internal opamp feature for reading the input, or does this require an external opamp chip?
(According the datasheet CH32V006F8P6 has a built-in OPA with 3 channels)
I'm still searching for some simple examples I can use to gain better understanding of using the OPA. Can you recommend any example code?

[boda-akos]

ch32v006_7segm_LED.zip

My example LED driver has timer interrupts and constant display segment current. Hope you will test it out.

About your question : no, I have not used the internal OPA. Not even tested yet... I recommend LM358 LMV358 OPA2171AIDR (good ->better->best) as amplifiers or followers in front of ADC inputs. The ADC of old Atmel AVRs represent a capacitive load, i.e. infinite input impedance, so one was getting lazy about that. I was surprised using the CH32 ADC and getting false readings that is why I started measuring the ADC inputs.

[maxint-rd]

My example LED driver has timer interrupts and constant display segment current. Hope you will test it out.

Thank you for sharing that code. When I find some time I'll fill a breadboard to give it a go. I think your timer-interrupt implementation will be a good approach to make my scavenged vape-displays show some actual data.

The ADC of old Atmel AVRs represent a capacitive load, i.e. infinite input impedance, so one was getting lazy about that. I was surprised using the CH32 ADC and getting false readings that is why I started measuring the ADC inputs.

Hmm... perhaps I've been subject of such laziness too. I've had two X033 chips destroyed in a boost converter project that included reading 12V-20V voltages through a voltage divider. If I understand you correctly the low input impedance can cause inaccurate readings. Could it also cause damage to the chip by passing too much current?

In another project I'm using the ADC for reading an LDR to adjust output levels of a TM1638 LED matrix. It only has 8 levels, so there is no need to be very accurate.

[boda-akos]

The input impedance will cause errors in the measurement , just imagine that you have a two resistor voltage divider calculated for your 20V range, that is if your ADC reference is 3.3V the divider is about 1/6. Now you take two resistors 51k / 10k for 1/6 divider. The input's 75 kohm will appear parallel to the 10 kOhm, so the ADC value will be 12% less. Other than that, we don't know how linear is the 75 kohm as a function of voltage on input etc., no data, otherwise it could be included in the divider calculations. Concerning the destruction of a chip, the catalog data 0.3-0.5V above/below Vcc/Vee is permitted. The chip has ESD diodes protecting the inputs, which are for short-term protection only and withstand 1 to 5 mA current (answer from AI) if the voltage goes beyond. I used to connect capacitors 2.2 to 10uF between input and GND, these smoothen out the spikes (if ADC speed is not an issue).

[maxint-rd]

Thank you for your clear elaborate explanation. I think I now have better understanding of how to deal with this.

I don't know yet how exactly the two chips were destroyed. Perhaps they just received a much too high voltage on pin.. (They are show very low resistance between VCC and GND and can only be used as a small heating element...)

[boda-akos]

Hello MaxInt-Rd , See here the explanation by AI China (DeepSeek). Probably point 2. is your case.-------------------------------------------------------- The protection diodes are designed for brief, high-voltage, low-energy events like ESD from human touch. They are NOT designed for continuous overvoltage or high-current conditions. Key Limitations: - Very Limited Current Sinking Capability: The protection diodes are extremely small. They can only safely handle a tiny amount of current, typically in the range of 1 mA to 5 mA (this is not explicitly stated in the CH32V003 datasheet but is a standard figure for MCUs). If you try to use them as part of your circuit's normal operation (e.g., to clamp an input to 3.3V from a 5V signal using a series resistor), you must ensure the current through the diode remains below this very low threshold. - Latch-Up Risk: If you inject significant current into the VDD rail via the upper diode, you can raise the local VDD voltage. Under the wrong conditions, this can trigger a latch-up event—a parasitic silicon-controlled rectifier (SCR) structure turns on, creating a low-impedance path between VDD and VSS. This can lead to catastrophic failure due to excessive heat and destruction of the metal layers, unless power is removed very quickly. - Not for Power Rails: The protection on I/O pins does not protect against overvoltage on the VDD or VSS pins themselves. Those require external protection components (like TVS diodes) if your application is exposed to such risks. Practical Design Implications and Recommendations - For ESD (Handling, Environment): The built-in protection is sufficient for most general-purpose applications. You do not need to add extra components for basic ESD robustness. - For Interfacing with Higher Voltages (e.g., 5V Logic): Do NOT directly connect a 5V output to a CH32V003 pin (which is rated for 3.3V VDD). The protection diode will clamp it, but likely well above its safe current limit. Instead, use a level shifter circuit (active or passive). A simple, effective, and common method is to use a series resistor to limit the current into the clamping diode. A resistor in the range of 3.3kΩ to 10kΩ will limit the current to a safe 0.5mA - 1.5mA when the 5V input is clamped to ~3.6V. This is a standard and reliable technique. - For Noisy or Harsh Environments (Industrial, Automotive): If the system is exposed to transients beyond basic ESD (like surges), you must add external protection. This includes: - TVS Diodes: Placed on sensitive lines to shunt large transients before they reach the MCU pin. - Series Resistors: To limit peak current. - RC Filters: To slow down the edge of a transient, making it easier for the internal diodes to clamp. Conclusion Yes, the CH32V00x inputs have ESD protection diodes. You can rely on them for protection against static electricity during normal handling. However, you must never consider them as a substitute for proper circuit design when dealing with continuous overvoltage conditions. Always respect the "Absolute Maximum Ratings" and use external components like series resistors or dedicated level shifters when interfacing with signals outside the MCU's VDD/VSS range. New chat On Sunday, August 24, 2025 at 07:13:17 PM GMT+2, Maxint R&D ***@***.***> wrote: maxint-rd left a comment (openwch/arduino_core_ch32#219) Thank you for your clear elaborate explanation. I think I now have better understanding of how to deal with this. I don't know yet how exactly the two chips were destroyed. Perhaps they just received a much too high voltage on pin.. (They are show very low resistance between VCC and GND and can only be used as a small heating element...) — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you were mentioned.Message ID: ***@***.***>