ST AN2595 Application note

AN2595

Application note

Designing an application with the ST10F27xZx devices

Introduction

This note gives advice on designing applications based on devices of the ST10F27xZx family that includes the ST10F276Z5, ST10F273Z4, ST10F272Z2 and ST10F271Z1.

Six topics are covered:

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

oscillator

■ Details on startup 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

July 2007 Rev 1 1/20

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Contents AN2595 - Application note

Contents

1 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1 Oscillator characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Startup time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 PCB layout for ST10F27x oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Oscillator and EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 32 kHz oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Port0 startup configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1 Port0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Port0 startup configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Filtering / decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Decoupling on V18 pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Decoupling on +5 V supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3 Filtering / EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4 Unused general purpose pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Special pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1 External access enable/standby voltage supply pin (EA/V

4.2 RPD pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

) . . . . . . . 14

STBY

5 Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Analog digital converter (ADC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.1 Voltage drop in the source resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.2 Poor charging of the ADC internal resistance . . . . . . . . . . . . . . . . . . . . . 15

6.3 Errors due to high frequencies from input signal . . . . . . . . . . . . . . . . . . . 16

6.4 Reducing ADC errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.5 V

power-up and power-down sequence . . . . . . . . . . . . . . . . . . . . . . 16

AREF

7 External memory interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8 Connecting to L4969 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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AN2595 - Application note Contents

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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List of tables AN2595 - Application note

List of tables

Table 1. gm values for different types of oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 2. Clock options on PORT0 P0H.5/.7 for devices with low-power oscillator. . . . . . . . . . . . . . 10

Table 3. Clock options on PORT0 P0H.5 /.7 for devices with wide-swing oscillator. . . . . . . . . . . . . 10

Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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AN2595 - Application note List of figures

List of figures

Figure 1. Oscillator characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 2. Example of layout for external crystal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3. Port0 pin assignment for power-on configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 4. Implementation of decoupling capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 5. Filter based on discrete components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 6. Source internal resistance errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 7. ST10F27x connected to L4969 and other SPI devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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Oscillator AN2595 - Application note

1 Oscillator

The ST10F27xZx 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:

● ST10F276Z5 and ST10F273Z4: wide-swing oscillator (4 to 12 MHz)

● ST10F272Z2 and ST10F271Z1: low-power oscillator (4 to 8 MHz)

Moreover, the ST10F27x provides a new feature when the real-time clock ( RTC) module is used and a reference clock is needed in Powe r Down mode. In this case, two possib le 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 Pow er Down is entered and the real-time cloc k continues counting using the main oscillator clock signal as reference.

Refer to the ST10F27xZx 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), the key items are described in the following section.

Figure 1. Oscillator characteristics

On-chip Oscillator circuit

XTAL1 XTAL2

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.

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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