Using a G4 with an Oscillator

[Tobe]

Im planing on using a Jo22 oscillator with 24 Mhz: https://www.jauch.com/downloadfile/5ef1edcfb7fd5cebb9856be49a6932f5e/jo22-1.8-3.3v.pdf%C2%A0

Are there any things i have to look out for?

I havent found a example circuit, and the datasheet of the oscillator speaks of different load capacitances. What is this capacitance?

 

 

[Peter BENSCH]

Well, the manufacturer of the oscillator should be able to answer your questions. However, oscillators are quite easy to use and only need to be connected to VCC, GND and the output to the OSC_IN. In addition, the enable pin must be connected, whereby I assume that with Jauch the oscillator is enabled with high at pin e/d and in standby with low.

As far as the load capacitance is concerned, this refers to the capacitance at the output. However, if you keep the connection to the OSC_IN very short, you should only add a few pF to the PCB capacitance of a few pF.

Hope that helps?

Regards
/Peter

[Tesla DeLorean]

>>I havent found a example circuit, and the datasheet of the oscillator speaks of different load capacitances. What is this capacitance?

Capacitance being the resistance-to-change.

Here it is talking about the input capacitance (load) presented by the MCU and wiring to it, that the oscillator expects to be driving against to get the prescribed frequency and slew. If the MCU doesn't present enough you can add capacitance in the form of a capacitor to ground on the HSE_IN signal.

You also on the STM32 side want to be using the HSE_BYPASS mode for a LVCMOS input rather than the side of a crystal.

[Tobe]

"You also on the STM32 side want to be using the HSE_BYPASS mode for a LVCMOS input rather than the side of a crystal."

What do you mean by that?

[Tesla DeLorean]

I mean you need to change the characteristics of the input / inverter feed back

/** @defgroup RCC_HSE_Config HSE Config
 * @{
 */
#define RCC_HSE_OFF 0x00000000U /*!< HSE clock deactivation */
#define RCC_HSE_ON RCC_CR_HSEON /*!< HSE clock activation */
#define RCC_HSE_BYPASS (RCC_CR_HSEBYP | RCC_CR_HSEON) /*!< External clock source for HSE clock */

In GPIO input mode the capacitance is nominally around 5 pF, and the high/low thresholds change.

[Peter BENSCH]

@Tesla DeLorean The load capacity is not a setpoint or minimum value, but the maximum capacitance that the output of the oscillator can drive. Hence my statement that the perhaps 3...5pF board capacitance + perhaps 2...4pF line capacitance can easily be driven by the oscillator without the need for a buffer in between.

Do you agree?

[AScha.3]

This is an active oscillator, generating 24MHz output; the load cap is the maximum load, it can drive within its spec, so dont worry about this; just place the Jo22 close the cpu and a 100nF cer. cap close to the Jo22 .

My extra recommendation is to put a serial damping resistor , about 51 ohm, in the 24M clock trace to the cpu, to prevent any ringing.

And enable pin to Vcc : (= high)

 

[Tobe]

Could you explain more, how you came to the conclusion, that a 51 ohm resistor would  prevent a potential ringing? What was the setup?

[AScha.3]

Just experience - believe it and try it or ...

- read about reflections, transmission line theory 

- ringing and reflections on pcb traces

- measure and test it (if you have broadband VNA analyzer etc at hand )