Maxim Integrated High-Speed Microcontroller - Manual (Page 63)

High-Speed Microcontroller User’s Guide
Rev: 062210 63 of 176
5. CPU TIMING
The timing of the high-speed microcontroller is the area with the greatest departure from the original
8051 series. This section will briefly explain the timing and also compare it to the original.
5.1 Oscillator
The high-speed microcontroller provides an on-chip oscillator circuit that can be driven by an external
crystal or by an off-chip TTL clock source. The oscillator circuit provides the internal clocking signals to
the on-chip CPU and I/O circuits.
Figure 5-1 shows the required connections for a crystal. In most cases, a crystal will be the preferred
clock source. For very low-power applications, a low frequency ceramic resonator may also be used. The
capacitors shown in
Figure 5-1 are typical values. If a resonator is used, higher capacitance such as 47pF
may be needed.
For higher frequency designs, an off-chip clock oscillator is preferred (
Figure 5-2 ). When using an off-
chip oscillator, the duty cycle becomes important. As nearly as possible, a 50% duty cycle should be
supplied.
5.2 XTAL1
This pin is the input to an inverting high gain amplifier. It also serves as the input for an off-chip
oscillator. Note that when using an off-chip oscillator, XTAL2 is left unconnected.
5.3 XTAL2
This pin is the output of the crystal amplifier. It can be used to distribute the clock to other devices on the
same board. If using a crystal, the loading on this pin should be kept to a minimum, especially capacitive
loading.
5.4 Oscillator Characteristics
The high-speed microcontroller was designed to operate with a parallel resonant AT cut crystal. The
crystal should resonate at the desired frequency in its primary or fundamental mode. The oscillator
employs a high gain amplifier to assure a clean waveform at high frequency. Due to the high-performance
nature of the product, both clock edges are used for internal timing. Therefore, the duty cycle of the clock
source is of importance. A crystal circuit will balance itself automatically. Thus, crystal users will not
need to take extra precautions concerning duty cycle.
5.5 Crystal Selection
The high-speed microcontroller family was designed to operate with fundamental mode crystals for
improved stability. Although most high-speed (i.e., greater than 25MHz) crystals operate from their third
overtone, fundamental mode crystals are available from most major crystal suppliers. Designers are
cautioned to ensure that high-speed crystals being specified for use in their application do operate at the
rated frequency in their fundamental mode. The use of a third overtone crystal will typically result in
oscillation rates at one-third the desired speed.