MCP3912 Oscillator - Page 1
Forum section: electronics / projects

author: Phaedo
author_rank: Contributor
timestamp: September 15, 2025, 11:16:27 am
content: Hello ,I am trying to interface an MCP3912 with an STM32F103. I am using a 3.2768 MHz crystal oscillator.I am unable to communicate with the MCP3912 using the SPI. I checked with a scope on the oscillator pins , but I am unable to see a waveform across the crystal. Whereas for the STM32 I am able to see a pure sine wave for a crystal of 8MHz.The eval board schematic for MCP3912 shows the caps on the oscillator at 0.1uF. I have tried 16pF. 22pf and 0.1uF. If anyone has used this AFE, would be happy to get some ideas.checkpoints !!Cheers !!

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 15, 2025, 12:09:56 pm
content: Quote from: Phaedo on September 15, 2025, 11:16:27 amno, 18pf or 22pf (@10MHz).QuoteI know nothing about this chip, but the fact that this specification exists means it can at least operate without a clock. It must be able to communicate.Therefore, in my opinion, the cause lies elsewhere, not in the clock.First, you should check that SPI is functioning. at least in the initial state, the registers are configured not to oscillate.Quotecheck the datasheet carefully.

author: Phaedo
author_rank: Contributor
timestamp: September 15, 2025, 01:57:58 pm
content: Hello,Thanks for the reply!!SPI is not functioning.ChatGPT opines thus :No Software Activation:Unlike some devices that require enabling or configuring a clock in firmware, the MCP3912’s oscillator runs as soon as power and connections are established.Communication via SPI depends on the clock running, so if the oscillator isn’t working, SPI won't function — but it’s a hardware issue, not a software setting.Ok let me try a 10MHz with a 22pF. In any case, the datasheet clearly states that valid crystal frequency is from 1MHz to 20MHz.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 15, 2025, 02:28:40 pm
content: Quote from: Phaedo on September 15, 2025, 01:57:58 pmHa! Solved!read the datasheet and my replies carefully.it is not oscillate until you set the registers.Don't use AI, read the datasheet yourself and understand it.you are being fooled by AI illusions.Connect a scope, sample, check the MOSI on the rising edge of the clock, memo, and make sure the bits are being output properly. This is the first thing to do.Whether you have an MCP3912 installed or not.Starting from the basics.There's no need for AI.Quotevague, but it seems that as long as SCK is input, the registers are set properly even if an external clock is not connected/set.(maybe?)it would be obvious. if the register could not be set, it would mean that the bare crystal could not be used.

author: Phaedo
author_rank: Contributor
timestamp: October 17, 2025 10:36:03 am
content: Ok, no AI , read the datasheet and solved the issue of the crystal oscillation.I have another query though.The datasheet says :In case of positive saturation (CHn+ – CHn- > VREF/1.5),the output is locked to 7FFFFF for 24-bit mode (7FFFfor 16-bit mode). In case of negative saturation(CHn+ – CHn- < -VREF/1.5), the output code is lockedto 800000 for 24-bit mode (8000 for 16-bit mode).Which means I would get a maximum number of counts for V = 1.2V/1.5 = 0.8V .When I physically check , I get a maximum counts of 32767 atCH0+–CH1- = 0.78V. I am feeding the milli voltage directly across these pins with a mV calibrator and verified with a DMM.However the datasheet sheet (1.1 Electrical Specifications) says the maximum allowed input range is 0.6V to -0.6V. If this limitation is obeyed it means we would loose the full input range of the ADC ?

author: squadchannel
author_rank: Frequent Contributor
timestamp: October 17, 2025 11:27:36 am
content: If you're asking about the maximum input voltage for Vref?read carefully.-0.6V to VDD+0.6V

author: Phaedo
author_rank: Contributor
timestamp: October 17, 2025 05:52:41 pm
content: Check attached page.