ST72C254G2 datasheet(106/135 Pages) STMICROELECTRONICS

106 / 135 page

ST72104G, ST72215G, ST72216G, ST72254G

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CLOCK CHARACTERISTICS (Cont’d)

13.5.4 RC Oscillators

The ST7 internal clock can be supplied with an RC

oscillator. This oscillator can be used with internal

or external components (selectable by option

byte).

Figure 65. Typical Application with RC oscillator

Figure 66. Typical Internal RC Oscillator

Figure 67. Typical External RC Oscillator

Notes:

1. Data based on characterization results.

2. Guaranteed frequency range with the specified CEX and REX ranges taking into account the device process variation.

Data based on design simulation.

3. Data based on characterization results done with VDD nominal at 5V, not tested in production.

4. REX must have a positive temperature coefficient (ppm/°C), carbon resistors should therefore not be used.

5. Important: when no external CEX is applied, the capacitance to be considered is the global parasitic capacitance which

is subject to high variation (package, application...). In this case, the RC oscillator frequency tuning has to be done by

trying out several resistor values.

Symbol

Parameter

Conditi ons

Min

Typ

Max

Unit

fOSC

Internal RC oscillator frequency 1)

see Figure 66

3.60

5.10

MHz

External RC oscillator frequency 2)

114

tSU(OSC)

Internal RC Oscillator Start-up Time 3)

2.0

External RC Oscillator Start-up Time 3)

REX=47KΩ, CEX=”0”pF

REX=47KΩ, CEX=100pF

REX=10KΩ, CEX=6.8pF

REX=10KΩ, CEX=470pF

1.0

6.5

0.7

3.0

REX

Oscillator external resistor 4)

see Figure 67

Ω

CEX

Oscillator external capacitor

0 5)

470

OSC1

OSC2

fOSC

CEX

REX

EXTERN AL RC

INTERNAL RC

VREF

VDD

Current copy

Voltage generator

CEX discharge

ST72XXX

5.5

VDD [V]

3.8

3.9

4.1

4.2

4.3

fosc [MHz]

-40

°C

+25

°C

+85

°C

+125

°C

6.8

100

270

470

Cex [pF]

fosc [MHz]

Rex=10KOhm

Rex=15KOhm

Rex=22KOhm

Rex=33KOhm

Rex=39KOhm

Rex=47KOhm

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