12.1
AON Power Source
The AON domain is continuously powered from an off-chip power source, either a regulated
power supply or a battery.
12.2
AON Clocking
The AON domain is clocked by the low-frequency clock,
lfclk
. The core domain’s Tilelink
peripheral bus uses the high-frequency
coreClk
. A HF-LF power-clock-domain crossing
(VCDC) bridges between the two power and clock domains.
An alternative low-frequency clock source can be provided via the
aon_lfaltclksel
and
aon_lfaltclk
pads.
12.3
AON Reset Unit
An AON reset is the widest reset on the FE310-G000, and resets all state except for the JTAG
debug interface.
An AON reset can be triggered by an external active-low reset pin (
erst_n
), or expiration of the
watchdog timer (
wdogrst
).
These sources provide a short initial reset pulse
frst
, which is extended by a reset stretcher to
provide the LFROSC reset signal
lfroscrst
and a longer stretched internal reset,
srst
.
The
lfroscrst
signal is used to initialize the ring oscillator in the LFROSC. This oscillator pro-
vides
lfclk
, which is used to clock the AON.
lfclk
is also used as the clock input to
mtime
in
the CLINT.
The
srst
strobe is passed to a reset synchronizer clocked by
lfclk
to generate
aonrst
, an
asychronous-onset/synchronous-release reset signal used to reset most of the AON block.
The "mostly off" (MOFF) resets
coreclkrst
and
corerst
are generated by the Power Manage-
ment Unit (PMU) state machine after
aonrst
is deasserted.
12.4
External Reset Circuit
The FE310-G000 can be reset by pulling down on the external reset pin (
erst_n
), which has a
weak pullup. An external power-on reset circuit consisting of a resistor and capacitor can be
provided to generate a sufficiently long pulse to allow supply voltage to rise and then initiate the
reset stretcher.
The external reset circuit can include a diode as shown to quickly discharge the capacitor after
the supply is removed to rearm the external power-on reset circuit.
Chapter 12 Always-On (AON) Domain
SiFive FE310-G000 Manual: v3p2
© SiFive, Inc.
Page 54
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