Espressif ESP32 Technical Reference Manual page 694

31 Low-Power Management (RTC_CNTL)
• Digital & analog power controller: generates actual power-gating/clock-gating signals for digital parts and
analog parts.
• Sleep & wakeup controller: handles the entry into & exit from the low-power mode.
• Timers: include RTC main timer, ULP coprocessor timer and touch timer.
• Low-Power processor and sensor controllers: include ULP coprocessor, touch controller, SAR ADC con-
troller, etc.
• Retention memory:
– RTC slow memory: an 8 KB SRAM, mostly used as retention memory or instruction & data memory
for the ULP coprocessor. The CPU accesses it through the APB, starting from address 0x50000000.
– RTC fast memory: an 8 KB SRAM, mostly used as retention memory. The CPU accesses it through
IRAM0/DRAM0. Fast RTC memory is about 10 times faster than the RTC slow memory.
• Retention registers: always-on registers of 8 x 32 bits, serving as data storage.
• RTC IO pads: 18 always-on analog pads, usually functioning as wake-up sources.
Espressif Systems
Figure 31-6. RTC Structure
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ESP32 TRM (Version 5.2)