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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, 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. Flash

ESP8285 has a built-in SPI flash to store user programs.

•

Memory size: 1 MB

•

SPI mode: Dual SPI

3.2. Clock

3.2.1. 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. Refer to Table 3-1 to measure the

frequency offset.

3.2.2. External Clock Requirements

An externally generated clock is available with the frequency ranging from 24 MHz to 52

MHz. The following characteristics are expected to achieve good performance of radio.

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

Frequency tolerance

ΔFXO

–15

15

ppm

Frequency vs temperature (–25°C ~ 75°C)

ΔFXO,Temp

–15

15

ppm

Table 3-2. External Clock Reference

Parameter

Symbol

Min

Max

Unit

Clock amplitude

VXO

0.8

1.5

Vpp

Espressif

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