9S12X Pierced Oscillator

[Vince71]

Can anyone please explain this?

Several 9S12X MCU datasheets (9S12XDP, 9S12XHZ and other) depict 2 types of Pierced Oscillator configurations possible for use in the 9S12X (and legacy 9S12) MCUs, the "Loop Controlled Pierced Oscillator" and the "Full Swing Pierced Oscillator". In the introduction overview of these datasheet, it is mentioned that the "Bias Resistor Rb" of the Pierced Oscillator is integrated in the MCU to relief the need of an external one. If so, how can we still differentiate between the "loop controlled" and the "Full Swing"? As far as I know, the architectural difference between the two types is actually the use of that "Bias Resistor". If it is already existing and connected internally, aren’t we strictly using the full swing oscillator in this case? Thank you guys,

Vincent

[Vince71]

Steve, I got it. Thank you very much for the explanation.

[Vince71]

Steve, thanks a lot for the reply.

I understand the difference. What I meant was that there is a Bias resistor Rb connected internally, so it is always there; therefore, I think the oscillator can only be in a full swing mode (as I understand it). It cannot be put in a loop controlled mode since there IS the Rb connected.

As far as I understand:

- The loop controlled oscillator passive feedback net is: 2 resistors (R1, R2)and a crystal

- The full swing oscillator passive feedback net is: R1, R2, Rs (current damping, unnecessary in high frequencies), Rb (bias resistor), and a crystal

The Rb is always there so how can we really say that we can chose the "loop control"!! is there a way to neglect that Rb that I am unaware of?

Thanks a lot for any help.

[Steve]

As described in the spec the feedback resistor is present internally for loop-control mode only. It is not present for full-swing because this mode has two purposes:

1/ Full-swing crystal

2/ External clock input

If the bias resistor were internal then there would be a constant load on the external clock which is not desirable. This is why you must add the bias resistor externally when using full-swing mode with a crystal.

[Steve]

The difference between the oscillators refers to the internal operating mode of the circuit. Basically, full swing forces the crystal to oscillate  between the supply rails of the part (2.5V). Loop-controlled mode controls the oscillation amplifier so that it provides a more sinusoidal and lower voltage signal . This  reduces the current used and improves the EMC behaviour of the oscillator.