Datasheet

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3. Functional Description
• RAM size < 50 kB, that is, when ESP8285 is working under the Station mode and
connects to the router, programmable space accessible in heap + data section is
around 50 kB.
• There is no programmable ROM in the SoC, therefore, user program must be stored
in a SPI flash.
3.1.3. Flash
ESP8285 has a built-in SPI flash to store user programs.
• Memory size: 1 MB
• SPI mode: Dual Out
3.2. AHB and AHB Blocks
The AHB block performs as an arbiter. It controls the AHB interfaces through the MAC,
SDIO (host) and CPU. Depending on the address, the AHB data requests can go into one
of the two slaves.
• APB block
•
Flash controller (usually for standalone applications)
Data requests to the memory controller are usually high speed requests, and requests to
the APB block are usually register access.
The APB block acts as a decoder that only accesses the programmable registers within the
main blocks of ESP8285. Depending on the address, the APB request can go to radio, SI/
SPI, SDIO (host), GPIO, UART, real-time clock (RTC), MAC or digital baseband.
3.3. Clock
3.3.1. High Frequency Clock
The high frequency clock on ESP8285 is used to drive both transmit and receive mixers.
This clock is generated from internal crystal oscillator and external crystal. The crystal
frequency ranges from 24 MHz to 52 MHz.
The internal calibration inside the crystal oscillator ensures that a wide range of crystals can
be used, nevertheless the quality of the crystal is still a factor to consider to have
reasonable phase noise and good Wi-Fi sensitivity. Please refer to Table 3-1 for measuring
the frequency offset.
Table 3-1. High Frequency Clock Specifications
Parameter
Symbol
Min
Max
Unit
Frequency
FXO
24
52
MHz
Loading capacitance
CL
-
32
pF
Espressif
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2017.05