ST AN2595 Application note
July 2007 Rev 1 1/20
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
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Contents AN2595 - Application note
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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
STBY
) . . . . . . . 14
4.2 RPD pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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
AREF
power-up and power-down sequence . . . . . . . . . . . . . . . . . . . . . . 16
7 External memory interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8 Connecting to L4969 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
AN2595 - Application note Contents
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9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
List of tables AN2595 - Application note
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List of tables
Table 1. g
m
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
AN2595 - Application note List of figures
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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
Oscillator AN2595 - Application note
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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
The resonator component can be a crystal or a ceramic resonator. It is represented as a
series resonant branch R
s
, L
s
, C
s
. The amplification ability of the oscillator inverter is
replaced by a negative resistance R
L
and the capacitance C
L
contains the C
1
and C
2
load
capacitances and the stray capacitance of the resonator.
g
m
On-chip Oscillator circuit
Resonator
C
1
C
2
XTAL1 XTAL2
C
s
C
L
R
L
L
s
Equivalent circuit
R
s
Resonator
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