Data Sheet

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3. Functional Description
According to our current version of SDK, SRAM space available to users is assigned as
below.
•
RAM size < 50 kB, that is, when ESP8266EX is working under the Station mode and
connects to the router, the maximum 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 below:
•
OTA disabled: 512 kB at least
• OTA enabled: 1 MB at least
3.2. Clock
3.2.1. 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 3-1 to measure the
frequency offset.
⚠ Notice:
SPI mode supported: Standard SPI, Dual SPI and Quad SPI. The correct SPI mode should be selected
when flashing bin files to ESP8266. Otherwise, the downloaded firmware/program may not be working
properly.
Table 3-1. High Frequency Clock Specifications
Parameter
Symbol
Min
Max
Unit
Frequency
FXO
24
52
MHz
Loading capacitance
CL
-
32
pF
Motional capacitance
CM
2
5
pF
Series resistance
RS
0
65
Ω
Espressif
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2020.10