afs600 Actel Corporation, afs600 Datasheet - Page 34

• Manufacturer Part Number: afs600
• Description: Actel Fusion Programmable System Chips Mixed-signal Family With Optional Arm Support
• Manufacturer: Actel Corporation

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.
Table 2-10 • Crystal Oscillator Mode Definition
Figure 2-17 • Crystal Oscillator Macro
Figure 2-18 • Crystal Oscillator: RC Time Constant Values vs. Frequency (typical)
2 -2 0
Mode
RC network (Mode 0)
Low gain (Mode 1)
Medium gain (Mode 2)
High gain (Mode 3)
Actel Fusion Programmable System Chips
1.00E-0.3
1.00E-0.4
1.00E-0.5
1.00E-0.6
1.00E-0.7
0.0
0.5
RC Time Constant Values vs. Frequency
1.0
XTL
SELMODE
RTCMODE[1:0]
MODE[1:0]
RTCMODE/MODE[1:0]
A d v a n c e d v 1 . 4
Frequency (MHz)
1.5
00
01
10
11
2.0
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
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
CLKOUT
2.5
(Figure
3.0
3.5
2-18).
4.0
Frequency Range
0.032 to 0.20 MHz
0.20 to 2.0 MHz
2.0 to 20.0 MHz
4.5
N/A
Table
2-10.