ST AN1544 Application note

AN1544

APPLICATION NOTE

Designing an Application with ST10F269

This note brings advices in designing applications based on ST10F269. It includes six mains items which are:

-Information and recommendations in the use of external resonator with the on-chip oscillator,

-Details on start-up configuration and necessary precautions,

-Filtering, decoupling and special pins use,

-Recommendations to reduce ADC conversion errors,

-Memory interface,

-Interfacing with the L4969 CAN interface.

June 2002

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TABLE OF CONTENTS		PAGE
1 -	OSCILLATOR .............................................................................................................	3
1.1 -	OSCILLATOR CHARACTERISTICS ..........................................................................	3
1.2 -	RECOMMANDED CRYSTALS / CERAMIC RESONATORS .....................................	4
1.2.1 -	Murata Resonators ......................................................................................................	4
1.2.2 -	AVX Ceramic Resonators............................................................................................	4
1.2.3 -	NDK Crystals ...............................................................................................................	4
1.3 -	START-UP TIME ........................................................................................................	5
1.3.1 -	Start-up Time Results ..................................................................................................	5
1.4 -	PCB LAYOUT FOR ST10F269 OSCILLATOR ...........................................................	5
1.5 -	OSCILLATOR AND EMC ...........................................................................................	5

2 -	PORT0 START-UP CONFIGURATION ......................................................................	6
2.1 -	PORT0 ........................................................................................................................	6
2.2 -	PORT0 START-UP CONFIGURATION ...................................................................	7
3 -	FILTERING / DECOUPLING.......................................................................................	8
3.1 -	DECOUPLING ON DC1 AND DC2 PINS ...................................................................	8
3.2 -	DECOUPLING ON +5V SUPPLY ...............................................................................	8
3.3 -	FILTERING / EMC ......................................................................................................	9
3.4 -	UNUSED PINS ...........................................................................................................	9
4 -	SPECIAL PINS............................................................................................................	10
4.1 -	EA EXTERNAL ACCESS ENABLE PIN .....................................................................	10
4.2 -	RPD PIN .....................................................................................................................	10
5 -	RESET.........................................................................................................................	10
6 -	ADC .............................................................................................................................	11
6.1 -	VOLTAGE DROP IN THE SOURCE RESISTANCE ..................................................	11
6.2 -	POOR CHARGING OF THE ADC INTERNAL RESISTANCE ...................................	11
6.3 -	ERRORS DUE TO HIGH FREQUENCIES FROM INPUT SIGNAL ...........................	12
6.4 -	REDUCING ADC ERRORS .......................................................................................	12
6.5 -	VAREF POWER-UP / DOWN SEQUENCE ...............................................................	12
7 -	EXTERNAL MEMORY INTERFACE ..........................................................................	12
8 -	CONNECTING TO L4969 ...........................................................................................	13
9 -	REVISION HISTORY ..................................................................................................	13
9.1 -	CREATION OF THE AN1544 ON THE 24TH OF MAY 2002 .....................................	13
9.2 -	REVISION OF THE AN1544 ON THE 24TH OF JUNE 2002 ....................................	13
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AN1544 - APPLICATION NOTE

1 - OSCILLATOR

The ST10F269 can run with an external clock connected to XTAL1 input pin of the oscillator inverter or with a clock signal generated by a resonator connected to XTAL1 / XTAL2 pins. Refer to the ST10F269 datasheet for the possible combinations. This chapter provides with detailled information on the use of the on-chip oscillator in conjunction with an external resonator.

1.1 - Oscillator Characteristics

Using an external resonator (crystal or ceramic resonator) although simple to implement needs few basic precautions. Referring to the schematic of the on-chip oscillator (Figure 1), the key items are described in the following section.

Figure 1 ST10 Oscillator Equivalent Schematic

On-chip Oscillator circuit

	gm
XTAL1	Resonator	XTAL2

C1										C2

Equivalent circuit

CL RL

Ls	Cs	Rs
	Resonator

The resonator component can be a crystal or a ceramic resonator. It is represented as a series resonant branch Rs, Ls, Cs. The amplification ability of the oscillator inverter is replaced by a negative resistance RL and the capacicatance CL contains the C1, C2 load capacitances and the stray capacitance of the resonator .

The load capacitors C1 and C2 transform the gain of the amplifier (gm) into a negative series resistance RL to compensate for the losses of the crystal.

The best frequency stability is obtained when C1 = C2. The oscillation occurs when the sum of RL and Rs (the series resistance of the crystal) is negative.

By choosing C1 = C2 = C, the minimal gain of the amplifier (gm) is exressed as following:

gmmin = Rs × C2 × ω2 = Rs × C2 × (2 × π × f )2

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

The minimum transconductance (gm) of the ST10F269 oscillator inverter is 5 mA/V so the minimun series resistance value can be easily choosen in the crystal provider catalog.

The oscillation stability mainly depends on external parameters so only the transconductance (gm) can be guaranteed and the start-up time value will be defined by measurement at the application level.

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AN1544 - APPLICATION NOTE

1.2 - Recommanded Crystals / Ceramic Resonators

The Figure 2 shows the components to add to ST10F269 oscillator. The value of those components (C1, C2, RF) are given in the tables of recommanded crystals and ceramic resonators.

Figure 2 Additional Components to Use with External Resonator

VDD

ST10F269

XTAL1 XTAL2

RF

GND

C1											C2

1.2.1 - Murata Resonators

These are the recommanded ceramic resonators from Murata :

Frequency [Hz]	Type	Part Number	C1 [pF]	C2 [pF]	RF [Ω]
4.0 M	SMD	CSTCR4M00G15A( )-R0	(39)	(39)	Open
4.0 M	SMD	CSTCC4M00G16A( )-R0	(47)	(47)	Open
8.0 M	SMD	CSTCE8M00G15A( )-R0	(33)	(33)	Open
8.0 M	SMD	CSTCC8M00G16A( )-R0	(47)	(47)	Open
10 M	SMD	CSTCE10M0G15A( )-R0	(33)	(33)	Open
10 M	SMD	CSTCC10M0G16A( )-R0	(47)	(47)	Open
12 M	SMD	CSTCE12M0G15A( )-R0	(33)	(33)	Open
16 M	SMD	CSTCV16M0X11Q( )-R0	(5)	(5)	Open
22 M	SMD	CSTCV22M0X11Q( )-R0	(5)	(5)	Open
24 M	SMD	CSTCV24M0X11Q( )-R0	(5)	(5)	Open
40 M	SMD	CSTCV40M0X11Q( )-R0	(5)	(5)	Open

For each of the ceramic resonators, Murata analysed :

–Oscillating frequency versus temperature,

–Oscillator start-up time, oscillating frequency, oscillating voltage, versus ST10F269 external supply voltage,

–Correlation between Murata standard test conditions (using 74HCU04) and ST10F269.

The reports are available on request to Murata.

1.2.2 - AVX Ceramic Resonators

The analysis is pending with AVX. This document will be updated as soon results are available.

1.2.3 - NDK Crystals

The analysis is pending with NDK. This document will be updated as soon results are available.

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1.3 - 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, drift).

Depending on applications requirements and possiblities, users can choose between short oscillator start-up time and frequency accuracy.

From ST10 perspective, the worst case for the oscillator start-up time is high temperature and low voltage.

1.3.1 - Start-up Time Results

The measurements done by Murata on ST10F269 and ceramic resonators show start-up times in the 0.05ms range.

1.4 - PCB Layout for ST10F269 Oscillator

The following figure shows the proposed layout for ST10F269 oscillator.

Figure 3 Example of Layout fot External Crystal

ST10F269

Decoupling capacitor

XTAL2

CD

XTAL1

V

DD

VSS

Vias to VDD

C2

C1

	Crystal
Vias to GND	Vias to GND
Crystal package	Ground plate
soldered to GND

1.5 - Oscillator and EMC

ST10F269/ST10F280 oscillator has an integrated gain control to minimize EMC and power consumption. This does not prevent users to check the following rules :

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

–Layout the ground supply on the basis of low impedance.

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

–Do not layout sensitive signals near the oscillator. Analyze cross-talk between different layers.

–VSS pin close to XTAL pins must be connected to the ground plane and decoupled to the closest VDD pin.

–Capacitors shall be placed at both ends of the crystal, directly connected to the ground plane while keeping the overall loop as small as possible.

–Crystal package, when metalic, shall be directly connected to the ground.

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