Device Architecture
Crystal Oscillator
The on-chip crystal oscillator circuit works with an off-chip crystal to generate a high-precision
clock. It has an accuracy of 100 ppm (0.01%) and is capable of providing system clocks for Fusion
peripherals and other system clock networks, both on-chip and off-chip. When combined with the
on-chip CCC/PLL blocks, a wide range of clock frequencies can be created to support various design
requirements.
The on-chip circuitry is designed to work with an external crystal, a ceramic resonator, or an RC
network. It can only support one of these configurations at a time. Typical design practices dictate
that the desired mode for the crystal oscillator be determined and the board designed for a single
configuration. The crystal oscillator supports four modes of operation, defined in Table 2-10.
In Mode 0, the oscillator is configured to work with an external RC network. The RC components
are connected to the XTAL1 pin, with XTAL2 left floating. The frequency generated by the circuit in
Mode 0 is determined by the RC time constant of the selected components (Figure 2-18).
Table 2-10 • Crystal Oscillator Mode Definition
Mode
RTCMODE/MODE[1:0]
Frequency Range
N/A
RC network (Mode 0)
Low gain (Mode 1)
Medium gain (Mode 2)
High gain (Mode 3)
00
01
10
11
0.032 to 0.20 MHz
0.20 to 2.0 MHz
2.0 to 20.0 MHz
XTL
CLKOUT
SELMODE
RTCMODE[1:0]
MODE[1:0]
Figure 2-17 • Crystal Oscillator Macro
RC Time Constant Values vs. Frequency
1.00E-0.3
1.00E-0.4
1.00E-0.5
1.00E-0.6
1.00E-0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Frequency (MHz)
Figure 2-18 • Crystal Oscillator: RC Time Constant Values vs. Frequency (typical)
2-22
Preliminary v1.7
ACTEL [ Actel Corporation ]
