Maxim Integrated Ultra-High-Speed Flash Microcontroller - Manual (Page 50)

5-3
XTAL1
XTAL2
18pF 18pF
ULTRA-HIGH-SPEED
MICROCONTROLLER
TO INTERNAL
CIRCUITS
XTAL1
XTAL2
ULTRA-HIGH-SPEED
MICROCONTROLLER
TO INTERNAL
CIRCUITS
CLOCK
OSCILLATOR
Figure 5-2. Clock Source Input
SECTION 5: CPU TIMING
The timing of the ultra-high-speed microcontroller is the area with the greatest departure from the original 8051 series. This section
explains the timing and compares it to the original 8051.
Oscillator
The ultra-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. In many designs, a
crystal is the preferred clock source. Figure 5-1 shows the required connections for a crystal and typical capacitor values. Some
designs may prefer using an off-chip clock oscillator as the primary clock source. This configuration is illustrated in Figure 5-2. When
using an off-chip oscillator, the duty cycle becomes important. As near as possible, a 50% duty cycle should be supplied.
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.
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 crys-
tal, the loading on this pin should be kept to a minimum, especially capacitive loading.
Oscillator Characteristics
The ultra-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 balances itself automatically. Thus crystal users do not need to take extra pre-
cautions concerning duty cycle.
Crystal Selection
The ultra-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.
Figure 5-1. Crystal Connection
Ultra-High-Speed Flash
Microcontroller User’s Guide
Maxim Integrated