S12XHZ: Pierce Oscillator Select (PE7/XCLKS)

[Vince71]

Hello all,

In 9S12XHZ MCUs (as well as other S12 and S12X MCUs), the pin PE7/XCLKS is used to select whether a Loop Contrlled or Full Swing Pierce oscillator is being used (PE = 0 --> Loop controled / PE = 1 --> full swing). Just would like to verify, can I use PE7 as an I/O after the MCU bootup and the oscillator start?

Thanks in advance for any help.

[kef]

Vince71,

yes, you can use XCLKS for I/O. But you should provide that at the time MCU resets (no matter is it COP, poweron or any other reset), your external hardware shouldn't disturb MCU sampling incorrect XCLKS pin state value. If oscilator is designed for loop-controlled Pierce, then MCU has to sample corresponding XCLKS value.

sfb,

In loop control mode, oscilator tracks amplitude of oscilation and adjusts the gain of oscilator amplifier in order to reduce emissions and power consumptions etc. Full swing is full swing, the gain of osc. amplifier is set probably to max, and you should tune the oscilator with parallel and series resistors. Advantage of loop controlled Pierce is, at least less components, no external oscilator resistors.

[Vince71]

Kef,

Thanks a lot for the clarification. So I think if I want to select the loop-control (PE7=0), I would pull down PE7 to ground with an appropriate resistor; and connect the periferral pin to PE7 but make sure not to put any input on it untill reset is done?

Sfb & Kef,

Kef or any other contributer, please correct me if I am wrong. Loop control Pierce is less EMI than full-swing because full swing is a square wave (hence unlimited number of harmonics), while loop control is almost a sine wave??

Also, full swing would allow bus frequency equal to crystal frequency (so for 40 MHz clk, we can use 40 MHz crystal), while in loop control, we need to use the PLL in order to get bus clk that is higher than 16 MHz crystal?

Thank you all.

[sfb]

Hello kef,

So there is no EMC immunity and reliability (and stability) differences both configurations?

[sfb]

And what is the difference between loop controlled and full swing?