ST AN2353 Application note

AN2353

Application note

Designing an application with the ST10F27x devices

Introduction

This note gives advice on designing applications based on devices of the ST10F27x family which includes the ST10F276, ST10F275, ST10F273, ST10F272 and ST10F271.

Six topics are covered:

■ Information and recommendations on using an external resonator with the on-chip

oscillator

■ Details on start-up configuration and necessary precautions

■ Filtering, decoupling and use of special pins

■ Recommendations to reduce ADC conversion errors

■ Memory interface

■ Interfacing with the L4969 CAN interface

April 2006 Rev 1 1/18

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Contents AN2353

Contents

1 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Oscillator characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Start-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 PCB la yout for SST10F27x oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Oscillator and EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5 32 kHz oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Port0 start-up configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Port0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Port0 start-up configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Filtering / decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Decoupling on V18 pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 Decoupling on +5V supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Filtering / EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Unused general purpose pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Special pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 External access enable/stand-by voltage supply pin (EA/VSTBY) . . . . . . 12

4.2 RPD Pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Analog Digital Converter (ADC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6.1 Volt age drop in the source resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6.2 Poor charging of the ADC internal resistance . . . . . . . . . . . . . . . . . . . . . 13

6.3 Errors due to high frequencies from input signal . . . . . . . . . . . . . . . . . . . 14

6.4 Reducing ADC errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.5 Varef po wer-up and power-down sequence . . . . . . . . . . . . . . . . . . . . . . . 14

7 External memory interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8 Connecting to L4969 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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AN2353 Contents

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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Oscillator AN2353

1 Oscillator

The ST10F27x can run with an external clock connected to the XTAL1 input pin of the oscillator inverter or with a clock signal generated by a resonator connected to the XTAL1 / XTAL2 pins. According to the device, two kinds of oscillators have been implemented:

● ST10F276E, ST10F275E and ST10F273E: Wide-swing oscillator

● ST10F272E/B and ST10F271E/B: Low-power oscillator

Moreover, the ST10F27x provides a new feature when the Real Time Clock module is used and a reference clock is needed in Power Down mode. In this case, two possible configurations may be selected by the user application according to the desired level of power reduction:

● A 32 kHz crystal is connected to the on-chip 32 kHz oscillator (pins XTAL3 / XTAL4)

and running. In this case, the main oscillator is stopped when Power Down mode is entered, while the Real Time Clock continues counting using the 32 kHz clock signal as reference.

● Only the main oscillator is running (XTAL1 / XTAL2 pins). In this case, the main

oscillator is not stopped when Power Down is entered and the Real Time Clock continues counting using the main oscillator clock signal as reference.

Refer to the ST10F27x datasheet for the possible combinations. This chapter provides detailed information on the use of the on-chip oscillator in conjunction with an external resonator.

1.1 Oscillator characteristics

Although simple to implement, using an external resonator (crystal or ceramic resonator) requires a few basic precautions. Referring to the schematic of the on-chip oscillator (

Figure 1

Figure 1. Oscillator characteristics

The resonator component can be a crystal or a ceramic resonator. It is represented as a series resonant branch R replaced by a negative resistance R capacitances and the stray capacitance of the resonator.

), the key items are described in the following section.

On-chip Oscillator circuit

XTAL1 XTAL2

Resonator

, Ls, Cs. The amplification ability of the oscillator inverter is

Equivalent circuit

Resonator

and the capacitance CL contains the C1 and C2 load

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AN2353 Oscillator

The load cap aci tor s C1 and C2 transf orm th e ga in o f t he am plif ier (gm) into a negative series resistance R

The best frequency stability is obtained when C of R

and Rs (the series resistance of the crystal) is negative.

By choosing C

to compensate for the losses of the crystal.

= C2. The oscillation occurs when the sum

= C2 = C, the minimal gain of the amplifier (gm) is expressed as follows:

min

Rs C

×ω

× Rs C2× 2 π× f×()

×==

The minimal gain of the amplifier also implicitly sets the on-chip oscillator start-up time.

Table 1. g

Symbol Parameter Version Conditions

values for different types of oscillators

Value

Unit

Min Typ Max

Oscillator transconductance

The oscillation stability mainly depends on external parameters so only the transconductance (g measurement at the application level.

1.2 Start-up time

Ceramic resonators have a much shorter start-up time than crystals (about 100 times faster) but have a lower accuracy on the frequency (initial tolerance, temperature variations and drift).

Depending on applications oscillator start-up time and frequency accuracy.

Fro m an S T 10 perspective, the w or s t ca se co nd i ti o n / environment for t h e o sc il l at or s tart-up time is high temperature and low voltage.

Wide-swing 8 17 35 Low-power 0.7 2.5 6

32 kHz

) can be guaranteed and the start-up time value is defined by

Start-up 20 31 50 Norm al run 8 17 30

requirements and possibilities, users can choose between short

mA/V

µ A/V

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Oscillator AN2353

1.3 PCB layout for ST10F27x oscillator

The following

Figure 2

shows the proposed layout for the ST10F27x oscillator.

Figure 2. Example of layout for external crystal

ST10F27x

Vias to VDD

Vias to GND

Crystal package

soldered to GND

1.4 Oscillator and EMC

Decoupling capacitor

XT AL2

Crystal

Ground plate

XTAL1

Vias to GND

The ST10F27x oscillator has an integrated gain control to minimize EMC and power consumption. However, this does not prev ent users from observing / respecting the following rules / recommendations:

● Avoid other high frequency signals near the oscillator circuitry . These can influence the

oscillator.

● Lay out/configure the ground supply on the basis of low impedance.

● Shield the crystal with an additional ground plane underneath the crystal.

● Do not lay out sensitive signals near the oscillator. Analyze cross-talk between different

layers.

● The V

pin close to the XTAL pins must be connected to the ground plane and

decoupled to the closest V

● Capacitors are placed at both ends of the crystal, directly connected to the ground

plane while keeping the overall loop as small as possible.

● The crystal package, when metallic, is directly connected to the ground.

1.5 32 kHz oscillator

The same recommendations are valid for the 32 kHz oscillator. In any case, when the 32 kHz oscillator amplifier is not used, to avoid spurious consumption, XTAL3 must be tied to ground while XTAL4 is left open. Moreover, bit OFF32 in the RTCCON register should be set. The 32 kHz oscillator can only be driven by an external crystal and not by a different clock source.

pin.

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