Data Sheet

3. Functional Description

• RAM size < 50 kB, that is, when ESP8266EX 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 an external SPI ﬂash.

3.1.3. External Flash

ESP8266EX uses external SPI ﬂash to store user programs, and supports up to 16 MB

memory capacity theoretically.

The minimum ﬂash memory of ESP8266EX is shown in Table 3-1.

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 ESP8266EX. 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 ESP8266EX 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. Refer to Table 3-2 to measure the

frequency offset.

Table 3-1. Minimum Flash Memory

OTA

Minimum Flash Memory

Disabled

512 kB

Enabled

1 MB

Espressif

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2017.04