topic ATD reference voltage in S12 / MagniV Microcontrollers

ATD reference voltage

thommes — Wed, 10 Oct 2012 08:05:45 GMT

Hello,

I want to use the ATD on the MC9S12D64 with an external reference voltage. I want to use the LM336-5.0 as a 5V reference voltage. It has an operating current up to 10mA. No i want to connect this voltage to the Vdda and the Vrh Pin, but I am not sure if this is possible. What is the highest current for these two Pins?

Is it the 2.5mA like the digital I/O's? Then i could use it this way, or not? I'm really new in building Hardware, so i would be very thankful for an answer.

Just if it is important: Vregen, Vddr,Vddx, BKGD and Reset i want to supply by a different voltageregulator.

This is what my Power supply looks like:

Sorry for my bad english. Thanks and regards,

Thomas

Re: ATD reference voltage

GordonD — Thu, 11 Oct 2012 18:12:26 GMT

Thomas,

The 2.5 mA you reference is the injection current limit (i.e. when an I/O pin exceeds VDD + 0.3V or VSS - 0.3V). As long as an input pin is within VDD + 0.3V or VSS - 0.3V it is very high impedance and will only draw leakage current levels (see specifications). For VRH, the MC9S12DJ64 Device User Guide — V01.20 in Table A-8 specifies 0.375 mA for one A/D block enabled and 0.750 mA for two A/D blocks enabled.

I would not recommend connecting the output of the LM336-5.0 to VDDA unless the main regulator has better than 2% tolerance. This is because of parameter ∆VDDX in Table A-4 which states that Voltage Difference VDDX to VDDR and VDDA is to be no more than +/- 0.1V.

Also, make sure you have the proper bypass capacitors on all of the supplies as detailed in Table 22-1.

Regards,

Gordon

Re: ATD reference voltage

thommes — Fri, 12 Oct 2012 07:17:23 GMT

Thank you Gordon,
so you would recommend, to connect the output of the 7805 to at least Vdda and only Vrh to the LM336-5.0? Then it could be possible that Vdda is smaller than Vrh, which would lead to an error, right? Or is the extern reference Voltage not necessary? Then i would just use the 7805 for everything (Vddr, Vddx, Vdda, Vrh, Vregen, BKGD and Reset). Which way would you prefer?

Re: ATD reference voltage

thommes — Fri, 12 Oct 2012 07:18:08 GMT

Sorry, thank you for your time and the answer.