In the MC68HC11 the operating current (Idd) is the sum of the component currents

                       Idd = Osc + CPU + Analog + STOP

If we hold all variables (except frequency) constant, then for a given part.....

     A.  The STOP (leakage) current is constant.
     B.  The Analog current (A/D etc.) is constant if disabled or running on
         the internal RC oscillator.
     C.  The CPU current is a linear function of frequency.
     D.  The crystal oscillator current is a function of many variables
         including the external components, PC board layout, etc.  and it is
         therefore impossible to state a general rule.  If the external network
         is designed improperly, then it is quite possible for a low frequency
         circuit to use more current than a good high frequency design.


The external network can (and should) be optimized.  This is particularly
important at low frequencies where the relitively high loss in the external
network is high.  If the amplitude of the signal at the input to the oscillator
(EXTAL) is small, then the oscillator Idd goes up.

The oscillator current has two components, the load current and the 'short'
current.  The load current is simply the current that is dissipated in the
devices connected to the output of the oscillator (XTAL).  The 'short' current
is the Vdd to Vss current which is the result of both the P- and N-channel
being turned on.  This happens whenever the input signal is between a solid
logic '1' and a solid logic '0'.  This is roughly Vdd-0.6 and Vss+0.6 Volts.
The lowest possible oscillator current occurs with no load with a clean Vdd to
Vss square wave at the input.  The current is a function of frequency because
the load is simply the pin capacitance.  This current is very repeatable.  In
fact, this is the way that the 'HC11 is tested (see the fine print at the bottom

of the page in the data book).

If the input signal spends a majority of each cycle in the high 'short' current
mode, then the Idd can be very high.  This is what happens in some low frequency

designs.  The feedback resistor provides the DC bias so that the quiesent point
of the input signal is in the center of the linear operating range (approx. mid
supply).  If the loss in the network is high, the amplitude of the sinusodial
signal will be small.  The oscillator may always be in the high current range
because it never reaches the point where one of the transistors is off.

Beleive it or not, it is possible to put a resistor in series with the external
network and actually improve the amplitude of the input signal.  The theory for
this takes is too long to include here.

The following is a simple 'tune-up' proceedure for determining the optimal value

for Rx through a very simple experiment.




The 'Tune-Up' Procedure

There is a very simple 'tune-up' procedure which can be used to optimize
a Pierce oscillator design using any type of crystal, at any frequency,
supply voltage, and temperature.  The result is an 'optimal' circuit
design which is stable, reliable, and low power.


 A.  Construct an oscillator using the components (crystal, capacitors,
     inverter, resistors) that you intend to use.  If possible, the final
     PC board should be used because the physical layout of the board,
     the type of sockets, etc. can affect the oscillator operation.

 B.  Substitute a variable resistor for Rx.

 C.  Insert a micro-ammeter in the supply line to monitor Idd.

 D.  Tie all unused inputs to supply (Vdd) or ground (Vss).  This prevents
     'floating' inputs which can use relatively large currents that will
     affect the Idd measurement.

     In large chips, such as micro-processors, the chip should be held in
     reset.  This prevents any internal clocking which would raise the
     Idd and affect the measurement.  In Motorola MCU's, Reset will force
     all I/O ports into inputs.  All of these should be tied off to avoid
     'floating' inputs.

 E.  Monitor Idd while varying Rx.  There should be a very slight dip in
     Idd.  This dip is very broad (low Q) and very shallow (only a few
     percent).  The dip is the optimal point of operation.

 F.  Remove the variable resistor and substitute the nearest fixed value.
     As the shape of the Idd curve suggests, the exact resistor value is
     not critical.  Tolerances of up to 20% can generally be used.


Note that the value of Rx also affects the operating frequency.  Usually
this is only a few ppm.  If a precise frequency is needed, then it will
be necessary to use the following sub-procedure...

     AA.  Adjust Rx for 'optimal' Idd.
     BB.  Adjust Cy to set the correct frequency.
     CC.  Go back to AA and repeat until no changes are necessary.








These are the definitions of the parameters..........



Crystal Parameter Definitions:


                               R1       C1      L               
                                        ||                       
                    O-----*--/\/\/\/----||----XXXXX---*-----O   
                          |             ||            |         
                          |                           |         
                          |                           |         
                          |             ||            |         
                          +-------------||------------+         
                                        ||                       
                                        C0                       







External Component Definitions:
                                    |\                               
                                    | \                               
                                    |  \                             
                                    |   \                             
                               -----|    O----*------>               
                               |    |   /     |                       
                               |    |  /      \                       
                               |    | /       / Rx                   
                               |    |/        \                       
                               |              /                       
                               |      Rb      |                       
                               *----/\/\/\----*                       
                               |              |                       
                               |       _      |                       
                               |      | |     |                       
                               *----| | | |---*                       
                               |      |_|     |                       
                               |              |                       
                          Cx =====          ===== Cy                 
                               |              |                       
                               |              |                       
                              ===            ===                     
                               -              -                       


Amplifier Parameter Definitions: 

                |                                    \                 
                |                     || Cio          \                 
                |          -----------||-----------    \               
                |         |           ||           |    \               
                |         |                        |     \             
                |         |     |\                 |      \             
                |         |     | \                |       \           
                |         |     |  \               |        \           
                |         |     |   \      Ro      |         \  --       
          >-----+---------*-----+ Av O---/\/\/\----*----------+|  |------> 
                |         |     |   /              |         /  --       
                |         |     |  /               |        /           
                |   Cin =====   | /         Cout =====     /           
                |         |     |/                 |      /             
                |         |                        |     /             
                |        ===                      ===   /               
                |         -                        -   /               
                |                                     /                 
                |                                    /                 
                |                                   /                   





Some Practical Design Tips

The following is a summary of some guidelines which may help in design.

 Rb:
    This resistor sets the bias point.  It also affects the amplifier
 gain by introducing DC negative feedback.  Therefore, it should be as
 large as practical.  Usually the limiting factors are the input leakage
 current and the printed circuit board contamination (solder resin,
 finger prints, moisture, etc.).

 Rx:
    This resistor isolates the crystal network from the amplifier output
 and provides a needed phase shift.  The R/C lowpass filter, formed by
 Rx and Cy, decrease the probability of spurious oscillation at high
 frequencies.  The value of Rx should be approximately equal to the
 effective series resistance of the crystal at the operation frequency
 (if both Cx+Cin and Cy are equal).  An increase in the value of Rx will
 decrease the amount of feedback and improve stability.

 Cx and Cy:
    These capacitors are part of the crystal load network.  Typically,

                             Cx = Cy = (2 x CL)








I have simulated a couple of cases in the 1 to 2 MHz range that you mentioned
in your bulliten board message.  They are both for an 'HC11 in a plastic DIP
with Vdd = 5.0 Volts at 25 deg C.

1.000 MHz     R1 = 341 Ohms     Rb = 10 MOhms     CL = 10 pF     Rx = 10 kOhms
2.000 MHz     R1 = 154 Ohms     Rb = 10 MOhms     CL = 30 pF     Rx =  1 kOhms

This was done using an experimental crystal oscillator simulation program that
I am working on.  I would be interested in hearing the results of your work so
that I can verify the accuracy of my program's results.




Ken Burch
11309 April Drive
Austin  TX  78753

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