Espressif ESP32 Technical Reference Manual page 30

1 System and Memory
1.3.2.6 DMA
DMA uses the same addressing as the CPU data bus to read and write Internal SRAM 1 and Internal SRAM 2.
This means DMA uses an address range of 0x3FFE_0000 ~ 0x3FFF_FFFF to read and write Internal SRAM 1
and an address range of 0x3FFA_E000 ~ 0x3FFD_FFFF to read and write Internal SRAM 2.
In the ESP32, 13 peripherals are equipped with DMA. Table
1.3.2.7 RTC FAST Memory
RTC FAST Memory is 8 KB of SRAM. It can be read and written by PRO_CPU only at an address range of
0x3FF8_0000 ~ 0x3FF8_1FFF on the data bus or at an address range of 0x400C_0000 ~ 0x400C_1FFF
on the instruction bus. Unlike most other memory regions, RTC FAST memory cannot be accessed by the
APP_CPU.
The two address ranges of PRO_CPU access RTC FAST Memory in the same order, so, for example, addresses
0x3FF8_0000 and 0x400C_0000 access the same word. On the APP_CPU, these address ranges do not
provide access to RTC FAST Memory or any other memory location.
1.3.2.8 RTC SLOW Memory
RTC SLOW Memory is 8 KB of SRAM which can be read and written by either CPU at an address range of
0x5000_0000 ~ 0x5000_1FFF. This address range is shared by both the data bus and the instruction bus.
1.3.3 External Memory
The ESP32 can access external SPI flash and SPI SRAM as external memory. Table
memories that can be accessed by either CPU at a range of addresses on the data and instruction buses. When
a CPU accesses external memory through the Cache and MMU, the cache will map the CPU's address to an
external physical memory address (in the external memory's address space), according to the MMU settings.
Due to this address mapping, the ESP32 can address up to 16 MB External Flash and 8 MB External SRAM.
Bus Type
Low Address
Data 0x3F40_0000
Data 0x3F80_0000
Bus Type
Low Address
Espressif Systems
Table 1-3. Module with DMA
UART0
SPI1
I2S0
SDIO Slave
BT
Table 1-4. External Memory Address Mapping
Boundary Address
High Address
0x3F7F_FFFF
0x3FBF_FFFF
Boundary Address
High Address
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1-3 lists these peripherals.
UART1 UART2
SPI2 SPI3
I2S1
SDMMC
EMAC
WIFI
Size Target
4 MB External Flash
4 MB External SRAM
Size Target
30
1-4 provides a list of external
Comment
Read
Read and Write
Comment
ESP32 TRM (Version 5.2)