
EM358x
48
Rev. 0.4
To conserver power, OSCRC can be turned of during deep sleep. This mode is known as deep sleep 2. Since the
OSCRC is disabled, the sleep timer and watchdog timer do not function and cannot wake the chip unless the low-
frequency 32.768 kHz crystal oscillator is used. Non-timer based wake sources continue to function. Once a wake
event does occur, OSCRC is restarted and comes back up.
5.5.4
RAM Retention in deep sleep
The RAM can optionally be configured using the RAM_RETAIN register to select banks of locations to be non-
volatile. In deep sleep those banks selected are powered by a low leakage internal regulator that remains on
during deep sleep, powered from the always-on supply.
The RAM_RETAIN[15:0] register acts as a bit map of 4k byte blocks whereby setting a bit to 1 indicates that a
bank is to be retained. The default condition of 0xFFFF retains all banks in the RAM.
The bits in RAM_RETAIN are arranged so that bit [0] sets the retention option for bank 0, addresses from
0x20000000 to 0x20000FFF, bit [1] for addresses 0x20001000 to 0x20001FFF, and so on up to bit [15] for
addresses 0x2000F000 to 0x2000FFFF. It is not necessary for retained banks to be contiguous. Some banks may
need to be retained for correct operation of the Ember stack and others may be defined according to the
application.
5.5.5
Use of debugger with sleep modes
The debugger communicates with the EM358x using the SWJ.
When the debugger is logically connected, the CDBGPWRUPREQ bit in the debug port in the SWJ is set, and the
EM358x will only enter deep sleep 0 (the Emulated Deep Sleep state). The CDBGPWRUPREQ bit indicates that
a debug tool is logically connected to the chip and therefore debug state may be in the system debug
components. To maintain the debug state in the system debug components only deep sleep 0 may be used, since
deep sleep 0 will not cause a power cycle or reset of the core domain. The CSYSPWRUPREQ bit in the debug
port in the SWJ indicates that a debugger wants to access memory actively in the EM358x. Therefore, whenever
the CSYSPWRUPREQ bit is set while the EM358x is awake, the EM358x cannot enter deep sleep until this bit is
cleared. This ensures the EM358x does not disrupt debug communication into memory.
Clearing both CSYSPWRUPREQ and CDBGPWRUPREQ allows the EM358x to achieve a true deep sleep state
(deep sleep 1 or 2). Both of these signals also operate as wake sources, so that when a debugger logically
connects to the EM358x and begins accessing the chip, the EM358x automatically comes out of deep sleep.
When the debugger initiates access while the EM358x is in deep sleep, the SWJ intelligently holds off the
debugger for a brief period of time until the EM358x is properly powered and ready.
Note:
The SWJ-DP signals CSYSPWRUPREQ and CDBGPWRUPREQ are only reset by a power-on-reset or a
debugger. Physically connecting or disconnecting a debugger from the chip will not alter the state of these
signals. A debugger must logically communicate with the SWJ-DP to set or clear these two signals.
For more information regarding the SWJ and the interaction of debuggers with deep sleep, contact customer
support for Application Notes and ARM
®
CoreSight
TM
documentation.
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