Microcontrollers
are designed for requiring the minimum amount of external circuiting for their
system clocks. Most microcontrollers are designed to be able to run at a very
wide range of frequencies. From literally D>C (no clock transitions at all)
to tens of megahertz. This is accomplished by the use of fully static logic and
memory designs internal to the microcontrollers.
are designed for requiring the minimum amount of external circuiting for their
system clocks. Most microcontrollers are designed to be able to run at a very
wide range of frequencies. From literally D>C (no clock transitions at all)
to tens of megahertz. This is accomplished by the use of fully static logic and
memory designs internal to the microcontrollers.
There are
three different methods used for providing a clock in a microcontroller, once
each has unique advantages, and disadvantages. The first method is using a
crystal, wired as shown in fig. 2.3
three different methods used for providing a clock in a microcontroller, once
each has unique advantages, and disadvantages. The first method is using a
crystal, wired as shown in fig. 2.3
This allows a
crystal with a very precisely specified frequency (typically with error rate of
100s of parts per million) to be used to drive the microcontroller. This level
of precision is required for interfacing with other devices. The value of the
capacitors used are specified by the microcontrollers manufacture for a
specific crystal frequency.
crystal with a very precisely specified frequency (typically with error rate of
100s of parts per million) to be used to drive the microcontroller. This level
of precision is required for interfacing with other devices. The value of the
capacitors used are specified by the microcontrollers manufacture for a
specific crystal frequency.
Sometimes a
very large (in the order of mega ohms) resistor across the clock 0 and clock 1
is required for the clock to run stably. Often a manufacturer will specify that
a variable capacitor is attached to clock 1 to allow the oscillator to be
“turned” to the exact frequency.
very large (in the order of mega ohms) resistor across the clock 0 and clock 1
is required for the clock to run stably. Often a manufacturer will specify that
a variable capacitor is attached to clock 1 to allow the oscillator to be
“turned” to the exact frequency.
As a rule of
thumb, less capacitance will give you a better waveform. If you put on
capacitances that are too large, your clock will degrade to the point where the
controller would not start up. If you are not sure about the capacitance values
to be used, take a look at the clock waveform using high-impedance probe with
an oscilloscope. You should see the waveform shown in fg. 2.4.2 below
thumb, less capacitance will give you a better waveform. If you put on
capacitances that are too large, your clock will degrade to the point where the
controller would not start up. If you are not sure about the capacitance values
to be used, take a look at the clock waveform using high-impedance probe with
an oscilloscope. You should see the waveform shown in fg. 2.4.2 below
are the number
of components that are required and the fragility of quartz crystal. Both of
these problems can be eliminated be using a ceramic resonator.
of components that are required and the fragility of quartz crystal. Both of
these problems can be eliminated be using a ceramic resonator.
A ceramic
resonator is much more resistant to physical shocks count from three devices to
one . ceramic resonators typically have a frequency accuracy of several
thousand PPM (roughly 0.5%).
resonator is much more resistant to physical shocks count from three devices to
one . ceramic resonators typically have a frequency accuracy of several
thousand PPM (roughly 0.5%).
The next type
of system clock is the “RC Oscillator”. This uses a method of rise/fall time
characteristic of an RC network to provide a repeatable delay as a clock.
of system clock is the “RC Oscillator”. This uses a method of rise/fall time
characteristic of an RC network to provide a repeatable delay as a clock.
