ST STM8A Installation and operating instructions

July 2009 Doc ID 15988 Rev 1 1/12
AN3006
Application note
EMC guidelines for STM8A microcontroller
Introduction
To meet the demand for more functionalities, better performance and reduced cost, the
semiconductor industry develops microcontrollers with both high density design technology
and higher clock frequencies. This has intrinsically increased potential noise emission and
noise sensitivity.
Therefore, application developers must now apply EMC “hardening” techniques in the
design of firmware and PCB layout at system level.
This application note is intended for application designers who need to reach the optimum
level of EMC performance. It gives microcontroller EMC features and compliance standards.
This document only concentrates on STM8A features and behaviors. For general
information on EMC for ST microcontrollers, refer to the “EMC design guide for ST
microcontrollers” application note (AN1709).
General recommendations for STM8A devices are summarized in the “Getting started with
the STM8A” application note (AN2677). This document also provides basic EMC
recommendations.
Related documents
AN1015: Software techniques for improving microcontroller EMC performance.
AN1709: EMC design guide for ST microcontrollers.
AN2677: Getting started with the STM8A.
RM0009: STM8A reference manual.
www.st.com

Table of contents AN3006
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Table of contents
1 STM8A: designed for EMC robustness . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Layout recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Two-layer board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Single layer board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 EMC features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Software recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Critical bytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2 Illegal opcodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3 Reset sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

AN3006 STM8A: designed for EMC robustness
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1 STM8A: designed for EMC robustness
Most STM8A product family applications require a high level of robustness. The STM8A
devices have been designed to meet this requirement with a cost effective approach.
The following built-in features have been implemented for this purpose:
●Independent watchdog (IWDG) and window watchdog (WWDG) to detect unexpected
code execution, application behavior or code corruption
●Clock security system (CSS) to monitor external clock failures
●POR/BOR to make sure the microcontroller is properly supplied
●Critical registers complemented in order to prevent corruption
●Data EEPROM and Flash program memory which are write protected and which can
only be unprotected by entering a key combination
●Built-in I/O protection diodes
All these features could not be efficient enough without following board design EMC
recommendations.

Power supply AN3006
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2 Power supply
The STM8A product family is designed for standard applications in the 3.3 V to 5 V
(+/-10%) supply voltage range.
All power pins must be connected: VDD/VSS, VDDA/VSSA, VDDIOx/VSSIOx.
The VCAP pin is the regulator output. It provides the 1.8 V supply to the core and the
peripherals. This pin must be decoupled with a 470 nF up to 1µF capacitor. If not, or if a
wrong capacitor value is connected, the reset pin could toggle continuously.
For products with separated VDD and VDDIO1, it is strongly recommended to connect both
pins and decouple them with a single capacitor of 100 nF.
VDDA and VDDIO2, when present, must be decoupled with a dedicated 100 nF capacitor.
When designing the layout, all decoupling capacitors (including VCAP capacitor) must be
connected as close as possible to the VDD, VDDA, VDDIOx and VCAP pins.
Power supply connections are shown in the following diagram.
Figure 1. Power supply connections
Main / Low power
regulator
CPU
RAM
Logic
Analog functions
IOsIOs
XTAL
Star connected
Analog
signal
VCAP
VDD/VDDIO1
VDDIO2
VSSIO2
VDDA
VSSA
OSCIN
OSCOUT
VDDIO
VSSIO
VSSIO
VDDIO
3.3V - 5V
VSS/VSSIO1
ai15330B

AN3006 Layout recommendations
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3 Layout recommendations
The schematic diagram shows the appropriate bill of materials but, for EMC performance,
special care should be taken for the ground plane layout, track width and length, etc.
It is important to pay special attention to:
●the ground plane below the device used as a shield
●the supplies which should run around the device towards the other components (to
avoid creating a second power supply way).
●the ground path between the capacitor-loaded resonator and VSS which should be as
short as possible.
3.1 Two-layer board
Due to the position of VDDIO2 and VSSIO2, a two-layer board design is recommended for 44-
pin package and above.
Figure 2. Top view of a 64-pin package layout
Note: Grey: bottom side
Dark grey: top side
VSS
VDD
Crystal
Supplies towards other components
1: RESET
2: OSCIN
3: OSCOUT
4: VSSIO1
5: VSS
6: VCAP
7: VDD
8: VDDIO1
19: VDDA
20: VSSA
39: VSSIO2
40: VDDIO2
ai15361
1

Layout recommendations AN3006
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Figure 3. Bottom view of a 64-pin package layout
Note: Grey: bottom side
Dark grey: top side
Crystal
ai15362

AN3006 Layout recommendations
Doc ID 15988 Rev 1 7/12
3.2 Single layer board
For 32-pin package and below, a single layer board can offer a very good level of EMC
performance.
Figure 4. Top view of a 32-pin package layout
VSS
VDD
Suppliestowardsother components
Crystal
1
1: RESET
2: OSCIN
3: OSCOUT
4: VSS
5: VCAP
6: VDD
7: VDDIO1
9: VDDA
10: VSSA ai15363B

EMC features AN3006
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4 EMC features
EMC features are specific to each device. Please refer to the Electrical features section of
the product datasheet for any information on these features.

AN3006 Software recommendations
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5 Software recommendations
The standard software techniques for improving EMC performance described in the AN1015
application note (“Software techniques for improving microcontroller EMC performance”)
also apply to the STM8A product family.
This section lists the STM8A specifications related to the software techniques written in the
AN1015. It gives the values for STM8A products, although it is strongly recommended to
read the AN1015 carefully in order to safely implement the software recommendations.
5.1 Critical bytes
It is recommended avoiding the following critical bytes in your code:
●0x8E: opcode for HALT (instruction used to enter low power modes)
●0x8F: opcode for WFI (instruction Wait For Interrupt)
●0x82, 0xAC: opcode for INT (instruction used to jump into an interrupt routine)
●0x8B: opcode for SWBRK (software breakpoint instruction used to stall the CPU in
debug mode)
These critical bytes can be generated by the linker when computing the address destination
of a branch instruction (JUMP or CALL) in absolute or relative addressing modes. To
remove such critical bytes you can simply insert a NOP instruction before the target
address. For further explanation, refer to the AN1015 application note.
5.2 Illegal opcodes
To better control unexpected jumps, it is recommended to fill unused memory areas with
TRAP opcode (0x83).
Besides, if you want to generate an illegal opcode reset, the simpler illegal opcodes are the
following: 0x05, 0x0B, 0x71 and 0x75.
With pre-opcode, the list is longer:
●0x7220..0x722F, 0x7231, 0x7232, 0x7235, 0x723B, 0x7241, 0x7242, 0x7245, 0x724B,
0x7251, 0x7252, 0x7255, 0x725B, 0x7261, 0x7262, 0x7265, 0x726B, 0x7280..0x728E,
0x7290..0x72A1, 0x72A3..0x72A8, 0x72AA..0x72AF, 0x72B1, 0x72B3..0x72B8,
0x72BA, 0x72BC..0x72BF, 0x72E0..0x72EF, 0x72F1, 0x72F3..0x72F8, 0x72FA,
0x72FC..0x72FF
●0x9000, 0x9003..0x900F, 0x9020..0x9027, 0x902A, 0x902B, 0x9030..0x903F, 0x9041,
0x9045, 0x904B, 0x9051, 0x9055,0x905B, 0x9061, 0x9065,0x906B, 0x9071, 0x9072,
0x9075, 0x907B, 0x9080..0x9084, 0x9086..0x9088, 0x908A..0x908F, 0x9090..0x9092,
0x9098..0x909D, 0x90A0..0x90A2, 0x9090A4..0x90A6, 0x90A8..0x90AD,
0x90B0..0x90B2, 0x90B4..0x90BD, 0x90C0..0x90C2, 0x90C4..0x90CD
●0x9100..0x915F, 0x9161, 0x9162, 0x9165, 0x916B,0x9170..0x91A6, 0x91A8..0x91AE,
0x91B0..0x91C2, 0x91C4..0x91CD, 0x91E0,0x91FF
●0x9200..0x922F, 0x9231, 0x9232, 0x9235, 0x923B, 0x9240..0x925F, 0x9261, 0x9262,
0x9265, 0x926B, 0x9270..0x928C, 0x928E..0x92A6, 0x92A8..0x92AB, 0x92AD,
0x92AE, 0x92B0..0x92BB, 0x92BE, 0x92BF, 0x92E0..0x92FF.

Software recommendations AN3006
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5.3 Reset sources
Reset sources that triggered a reset can be checked in the Reset status register RST_SR
(at address 0x50B3). The different reset sources which can be identified through the Reset
status register are:
●EMC reset,
●SWIM reset,
●Illegal opcode reset,
●Independent watchdog reset,
●Window watchdog reset.
Each of these flags can be cleared by software by writing 1 into the corresponding bit
location. They can also all be cleared by a power-on reset.
For a description of these reset sources, please refer to the STM8A microcontroller family
reference manual (RM0009).