MCX MicrocontrollersのトピックRe: Voltage specification for externally provided clock signal (MCX A185 VLH)

Voltage specification for externally provided clock signal (MCX A185 VLH)

wyss-11 — Thu, 18 Jun 2026 13:17:10 GMT

I can not find the max. voltage allowed at the EXTAL48M pin for the MCXA185VLH MCU in the datasheets.

I know from the LPCxxx families of microcontrollers that the oscillator input is very picky about this amplitude (0.85Vpp or similar).

Can I feed an externally generated 3.3V oscillator clock signal directly to the EXTAL48M pin?

Re: Voltage specification for externally provided clock signal (MCX A185 VLH)

carlos_o — Thu, 18 Jun 2026 21:02:14 GMT

Hi @wyss-11

Thanks for your post!

MCXA18 does have an EXTAL48M input voltage definition, but it is specified indirectly through the standard input thresholds VIH/VIL relative to VDD, not as a separate fixed EXTAL48M voltage value.

The datasheet only stands the Peak-to-peak amplitude of oscillation as typical value of 0.6V.

When VDD is 3.3 V, EXTAL48M can be driven by a 3.3 V external oscillator as long as the clock meets the VIH/VIL thresholds and does not exceed the VDD + 0.3 V input limit.

Re: Voltage specification for externally provided clock signal (MCX A185 VLH)

wyss-11 — Fri, 19 Jun 2026 08:36:19 GMT

Thanks for your post.

Yes, the datasheet specifies Vec (Externally provided input clock amplitude) with max. voltage as VIH. But as VIH is 0.7xVDD, this would mean that with VDD=3.3V, Vec max. would be 2.31V only.

So either this datasheet specification is wrong, or Vec max. is really 2.31V in case of VDD=3.3V?

Re: Voltage specification for externally provided clock signal (MCX A185 VLH)

carlos_o — Fri, 19 Jun 2026 17:52:05 GMT

Hi @wyss-11

The VEC specification refers to the minimum voltage levels required by the external oscillator input to recognize valid low and high states.

When VDD = 3.3 V, 2.31 V is the minimum voltage that will be detected as a valid high level.