XS1-U8A-64-FB96 Datasheet 23
The device is quiescent in the ASLEEP state, and is running in the AWAKE state. The
other states allow a controlled transition between AWAKE and ASLEEP.
A transition from AWAKE state to ASLEEP state is instigated by a sleep request:
either a write to the general control register or from the USB block requesting entry
to standby mode. Sleep requests must only be made in the AWAKE state.
A transition from the ASLEEP state into the AWAKE state is instigated by a wakeup
request triggered by a request from the USB block to exit standby mode an input,
or a timer. The device only responds to a wakeup stimulus in the ASLEEP state. If
wakeup stimulus occurs whilst transitioning from AWAKE to ASLEEP, the appropriate
response occurs when the ASLEEP state is reached.
Configuration is through a set of registers documented in Appendix K.
14.3 Deep Sleep Modes and Real-Time Counter
The normal mode in which the XS1-U8A-64-FB96 operates is the AWAKE mode. In
this mode, all cores, memory, and peripherals operate as normal. To save power,
the XS1-U8A-64-FB96 can be put into a deep sleep mode, called ASLEEP, where
the digital node is powered down, and most peripherals are powered down. The
XS1-U8A-64-FB96 will stay in the ASLEEP mode until one of three conditions:
1. An external pin is asserted or deasserted (set by the program);
2. The 64-bit real-time counter reaches a value set by the program; or
3. The USB host (if USB is enabled) performs a wakeup.
When the chip is awake, the real-time counter counts the number of clock ticks
on the oscillator. As such, the real-time counter will run at a fixed ratio, but
synchronously with the 100 MHz timers on the xCORE Tile. When asleep, the
real-time counter can be automatically switched to the 31,250 Hz silicon oscillator
to save power (see Appendix I). To ensure that the real-time counter increases
linearly over time, a programmable value is added to the counter on every 31,250
Hz clock-tick. This means that the clock will run at a granularity of 31,250 Hz
but still maintain real-time in terms of the frequency of the main oscillator. If an
accurate clock is required, even whilst asleep, then an external crystal or oscillator
shall be provided that is used in both AWAKE and ASLEEP state.
The designer has to make a trade-off between accuracy of clocks when asleep
and awake, costs, and deep-sleep power consumption. Four example designs are
shown in Figure 15.
Clocks used Power BOM Accuracy
Awake Asleep Asleep costs Awake Asleep
20 Mhz SiOsc 31,250 SiOsc lowest lowest lowest lowest
24 MHz Crystal 31,250 SiOsc lowest medium highest lowest
5 MHz ext osc 5 MHz ext osc medium highest highest highest
24 MHz Crystal 24 MHz crystal highest medium highest highest
Figure 15:
Example
trade-offs in
oscillator
selection
XM002429, XS1-U8A-64-FB96