User's Manual

3 FUNCTIONAL DESCRIPTION
3. Functional Description
This chapter describes the modules and functions implemented in ESP-WROOM-32.
3.1 CPU and Internal Memory
ESP32 contains two low-power Xtensa
®
32-bit LX6 microprocessors. The internal memory includes:
• 448 KBytes ROM for booting and core functions.
• 520 KBytes on-chip SRAM for data and instruction.
• 8 KBytes SRAM in RTC, which is called RTC SLOW Memory and can be accessed by the co-processor
during the Deep-sleep mode.
• 8 KBytes SRAM in RTC, which is called RTC FAST Memory and can be used for data storage and accessed
by the main CPU during RTC Boot from the Deep-sleep mode.
• 1 Kbit of EFUSE, of which 256 bits are used for the system (MAC address and chip configuration) and the
remaining 768 bits are reserved for customer applications, including Flash-Encryption and Chip-ID.
3.2 External Flash and SRAM
ESP32 supports 4 x 16 MBytes of external QSPI flash and SRAM with hardware encryption based on AES to
protect developer’s programs and data.
ESP32 accesses external QSPI flash and SRAM by the high-speed caches. Up to 16 MBytes of external flash are
memory mapped into the CPU code space, supporting 8, 16 and 32-bit access. Code execution is supported.
Up to 8 MBytes of external SRAM are memory mapped into the CPU data space, supporting 8, 16 and 32-bit
access. Data read is supported on the flash and SRAM. Data write is supported on the SRAM.
3.3 Crystal Oscillators
The frequencies of the main crystal oscillator supported include 40 MHz, 26 MHz and 24 MHz. The accuracy of
crystal oscillators applied should be ±10 PPM, and the operating temperature range -40°C to 85°C.
When using the downloading tools, remember to select the right crystal oscillator type. In circuit design, capacitors
C1 and C2 that connect to the earth, are added to the input and output terminals of the crystal oscillator respectively.
The values of the two capacitors can be flexible, ranging from 6 pF to 22 pF. However, the specific capacitive values
of C1 and C2 depend on further testing and adjustment of the overall performance of the whole circuit. Normally,
the capacitive values of C1 and C2 are within 10 pF if the crystal oscillator frequency is 26 MHz, while 10 pF<C1
and C2<22 pF if the crystal oscillator frequency is 40 MHz.
The frequency of the RTC crystal oscillator is typically 32 kHz or 32.768 kHz. The accuracy can be out of the range
of ±20 PPM, since the internal calibration is applied to correct the frequency offset. When the chip operates in low
power modes, the application chooses the external low speed (32 kHz) crystal clock rather than the internal RC
oscillators to achieve the accurate wakeup time.
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