ST STM32F20 Series Installation and operating instructions

August 2011 Doc ID 18267 Rev 2 1/29
AN3320
Application note
Getting started with STM32F20xxx/21xxx MCU
hardware development
Introduction
This application note is intended for system designers who require a hardware
implementation overview of the development board features such as the power supply, the
clock management, the reset control, the boot mode settings and the debug management. It
shows how to use the high-density performance line STM32F20xxx/21xxx product families
and describes the minimum hardware resources required to develop an
STM32F20xxx/21xxx application.
Detailed reference design schematics are also contained in this document with descriptions
of the main components, interfaces and modes.
www.st.com

Contents AN3320
2/29 Doc ID 18267 Rev 2
Contents
1 Power supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.1 Independent A/D converter supply and reference voltage . . . . . . . . . . . . 7
1.1.2 Battery backup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.3 Voltage regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Power supply schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Reset & power supply supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3.1 Power on reset (POR) / power down reset (PDR) . . . . . . . . . . . . . . . . . . 9
1.3.2 Programmable voltage detector (PVD) . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.3 System reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Clocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 HSE OSC clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 External source (HSE bypass) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 External crystal/ceramic resonator (HSE crystal) . . . . . . . . . . . . . . . . . 13
2.2 LSE OSC clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 External source (LSE bypass) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.2 External crystal/ceramic resonator (LSE crystal) . . . . . . . . . . . . . . . . . . 14
2.3 Clock security system (CSS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Boot configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Boot mode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Boot pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3 Embedded boot loader mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4 Debug management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2 SWJ debug port (serial wire and JTAG) . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3 Pinout and debug port pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1 SWJ debug port pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.2 Flexible SWJ-DP pin assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3.3 Internal pull-up and pull-down resistors on JTAG pins . . . . . . . . . . . . . . 19
4.3.4 SWJ debug port connection with standard JTAG connector . . . . . . . . . 20

AN3320 Contents
Doc ID 18267 Rev 2 3/29
5 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1 Printed circuit board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 Component position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3 Ground and power supply (VSS, VDD) . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.4 Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.5 Other signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.6 Unused I/Os and features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6 Reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.1 Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.2 Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.3 Boot mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.4 SWJ interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.5 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 Component references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

List of tables AN3320
4/29 Doc ID 18267 Rev 2
List of tables
Table 1. Boot modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 2. Debug port pin assignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 3. SWJ I/O pin availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 4. Mandatory components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Table 5. Optional components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Table 6. Reference connection for all packages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

AN3320 List of figures
Doc ID 18267 Rev 2 5/29
List of figures
Figure 1. Power supply overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. Power supply scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 3. Power-on reset/power-down reset waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 4. PVD thresholds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 5. Reset circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 6. HSE external clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 7. HSE crystal/ceramic resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 8. LSE external clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 9. LSE crystal/ceramic resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 10. Boot mode selection implementation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 11. Host-to-board connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 12. JTAG connector implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 13. Typical layout for VDD/VSS pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 14. STM32F207IG(H6) microcontroller reference schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Power supplies AN3320
6/29 Doc ID 18267 Rev 2
1 Power supplies
1.1 Introduction
The device requires a 1.8 V to 3.6 V operating voltage supply (VDD), excepted the WLCSP
package witch requires 1.65 V to 3.6 V. An embedded regulator is used to supply the
internal 1.2 V digital power.
The real-time clock (RTC) and backup registers can be powered from the VBAT voltage when
the main VDD supply is powered off.
Figure 1. Power supply overview
1. VDDA and VSSA must be connected to VDD and VSS, respectively.
2. The voltage on VREF ranges from 1.65 V to VDDA for WLCSP64+2 packages.
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AN3320 Power supplies
Doc ID 18267 Rev 2 7/29
1.1.1 Independent A/D converter supply and reference voltage
To improve conversion accuracy, the ADC has an independent power supply that can be
filtered separately, and shielded from noise on the PCB.
●the ADC voltage supply input is available on a separate VDDA pin
●an isolated supply ground connection is provided on the VSSA pin
When available (depending on package), VREF– must be tied to VSSA.
On 100-pin package and above and on WLCSP64+2
To ensure a better accuracy on low-voltage inputs, the user can connect a separate external
reference voltage ADC input on VREF+. The voltage on VREF+ may range from 1.8 V to
VDDA. On WLCSP64+2, the VREF- pin is not available, it is internally connected to the ADC
ground (VSSA).
On 64-pin packages
The VREF+ and VREF- pins are not available, they are internally connected to the ADC
voltage supply (VDDA) and ground (VSSA).
1.1.2 Battery backup
To retain the content of the Backup registers when VDD is turned off, the VBAT pin can be
connected to an optional standby voltage supplied by a battery or another source.
The VBAT pin also powers the RTC unit, allowing the RTC to operate even when the main
digital supply (VDD) is turned off. The switch to the VBAT supply is controlled by the power
down reset (PDR) circuitry embedded in the Reset block.
If no external battery is used in the application, it is highly recommended to connect VBAT
externally to VDD.
1.1.3 Voltage regulator
The voltage regulator is always enabled after reset. It works in three different modes
depending on the application modes.
●in Run mode, the regulator supplies full power to the 1.2 V domain (core, memories
and digital peripherals)
●in Stop mode, the regulator supplies low power to the 1.2 V domain, preserving the
contents of the registers and SRAM
●in Standby mode, the regulator is powered down. The contents of the registers and
SRAM are lost except for those concerned with the Standby circuitry and the Backup
domain.
Note: Depending on the selected package, there are specific pins that should be connected either
to VSS or VDD to activate or deactivate the voltage regulator. Refer to section "Voltage
regulator" in STM32F20xxx/21xxx datasheet for details.

Power supplies AN3320
8/29 Doc ID 18267 Rev 2
1.2 Power supply schemes
The circuit is powered by a stabilized power supply, VDD.
●Caution:
– The VDD voltage range is 1.8 V to 3.6 V (and 1.65 V to 3.6 V for WLCSP64+2
package)
●The VDD pins must be connected to VDD with external decoupling capacitors: one
single Tantalum or Ceramic capacitor (min. 4.7 µF typ.10 µF) for the package + one
100 nF Ceramic capacitor for each VDD pin.
●The VBAT pin can be connected to the external battery (1.65 V < VBAT < 3.6 V). If no
external battery is used, it is recommended to connect this pin to VDD with a 100 nF
external ceramic decoupling capacitor.
●The VDDA pin must be connected to two external decoupling capacitors (100 nF
Ceramic + 1 µF Tantalum or Ceramic).
●The VREF+ pin can be connected to the VDDA external power supply. If a separate,
external reference voltage is applied on VREF+, a 100 nF and a 1 µF capacitors must be
connected on this pin. In all cases, VREF+ must be kept between 1.65 V and VDDA.
●Additional precautions can be taken to filter analog noise:
–V
DDA can be connected to VDD through a ferrite bead.
–TheV
REF+ pin can be connected to VDDA through a resistor (typ. 47 Ω).
●For the voltage regulator configuration, there are specific pins (REGOFF and IRROFF
depending on the package) that should be connected either to VSS or VDD to activate
or deactivate the voltage regulator specific. Refer to section "Voltage regulator" in
STM32F20xxx/21xxx datasheet for details.
●When the voltage regulator is enabled, VCAP1 and VCAP2 pins must be connected to
2*2.2 µF Ceramic capacitor.

AN3320 Power supplies
Doc ID 18267 Rev 2 9/29
Figure 2. Power supply scheme
1. Optional. If a separate, external reference voltage is connected on VREF+, the two capacitors (100 nF and
1 µF) must be connected.
2. VREF+ is either connected to VREF or to VDDA.
3. N is the number of VDD and VSS inputs.
4. Refer to section "Voltage regulator" in STM32F20xxx/21xxx datasheet to connect REGOFF and IRROFF
pins.
1.3 Reset & power supply supervisor
1.3.1 Power on reset (POR) / power down reset (PDR)
The device has an integrated POR/PDR circuitry that allows proper operation starting from
1.8 V.
The device remains in the Reset mode as long as VDD is below a specified threshold,
VPOR/PDR, without the need for an external reset circuit. For more details concerning the
power on/power down reset threshold, refer to the electrical characteristics in
STM32F20xxx/21xxx datasheets.
On WLCSP66 package if IRROFF pin is set to VDD (in that case REGOFF pin must not be
activated, refer to section "Voltage regulator" in STM32F20xxx/21xxx datasheet for details ),
the PDR is not functional. Then the VDD can lower below 1.8 V, but the external circuitry
must ensure that reset pin is activated when VDD/VDDA becomes below 1.65 V.
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Power supplies AN3320
10/29 Doc ID 18267 Rev 2
Figure 3. Power-on reset/power-down reset waveform
1. tRSTTEMPO is approximately 2.6 ms. VPOR/PDR rising edge is 1.74 V (typ.) and VPOR/PDR falling edge is
1.70 V (typ.). Refer to STM32F20xxx/21xxx datasheets for actual value.
1.3.2 Programmable voltage detector (PVD)
You can use the PVD to monitor the VDD power supply by comparing it to a threshold
selected by the PLS[2:0] bits in the Power control register (PWR_CR).
The PVD is enabled by setting the PVDE bit.
A PVDO flag is available, in the Power control/status register (PWR_CSR), to indicate
whether VDD is higher or lower than the PVD threshold. This event is internally connected to
EXTI Line16 and can generate an interrupt if enabled through the EXTI registers. The PVD
output interrupt can be generated when VDD drops below the PVD threshold and/or when
VDD rises above the PVD threshold depending on the EXTI Line16 rising/falling edge
configuration. As an example the service routine can perform emergency shutdown tasks.
Figure 4. PVD thresholds
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AN3320 Power supplies
Doc ID 18267 Rev 2 11/29
1.3.3 System reset
A system reset sets all registers to their reset values except for the reset flags in the clock
controller CSR register and the registers in the Backup domain (see Figure 1).
A system reset is generated when one of the following events occurs:
1. A low level on the NRST pin (external reset)
2. window watchdog end-of-count condition (WWDG reset)
3. Independent watchdog end-of-count condition (IWDG reset)
4. A software reset (SW reset)
5. Low-power management reset
The reset source can be identified by checking the reset flags in the Control/Status register,
RCC_CSR.
The STM32F20xxx/21xxx does not require an external reset circuit to power-up correctly.
Only a pull-down capacitor is recommended to improve EMS performance by protecting the
device against parasitic resets. See Figure 5.
Charging and discharging a pull-down capacitor through an internal resistor increases the
device power consumption. The capacitor recommended value (100 nF) can be reduced to
10 nF to limit this power consumption;
Figure 5. Reset circuit
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Clocks AN3320
12/29 Doc ID 18267 Rev 2
2 Clocks
Three different clock sources can be used to drive the system clock (SYSCLK):
●HSI oscillator clock (high-speed internal clock signal)
●HSE oscillator clock (high-speed external clock signal)
●PLL clock
The devices have two secondary clock sources:
●32 kHz low-speed internal RC (LSI RC) that drives the independent watchdog and,
optionally, the RTC used for Auto-wakeup from the Stop/Standby modes.
●32.768 kHz low-speed external crystal (LSE crystal) that optionally drives the real-time
clock (RTCCLK)
Each clock source can be switched on or off independently when it is not used, to optimize
the power consumption.
Refer to the STM32F20xxx/21xxx reference manual RM0033 for the description of the clock
tree.
2.1 HSE OSC clock
The high-speed external clock signal (HSE) can be generated from two possible clock
sources:
●HSE external crystal/ceramic resonator (see Figure 7)
●HSE user external clock (see Figure 6)
1. The value of REXT depends on the crystal characteristics. Typical value is in the range of 5 to 6 RS
(resonator series resistance).
2. Load capacitance CLhas the following formula: CL= CL1 x CL2 / (CL1 + CL2) + Cstray where: Cstray is the pin
capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please
refer to Section 5: Recommendations on page 21 to minimize its value.
Figure 6. HSE external clock Figure 7. HSE crystal/ceramic
resonators
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AN3320 Clocks
Doc ID 18267 Rev 2 13/29
2.1.1 External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency from 1 to
16 MHz (refer to STM32F20xxx/21xxx datasheets for actual max value).
The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to
drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state (see
Figure 7 and Figure 6).
2.1.2 External crystal/ceramic resonator (HSE crystal)
The external oscillator frequency ranges from 4 to 26 MHz.
The external oscillator has the advantage of producing a very accurate rate on the main
clock. The associated hardware configuration is shown in Figure 7. Using a 25 MHz
oscillator frequency is a good choice to get accurate Ethernet, USB OTG high-speed
peripheral, and I2S.
The resonator and the load capacitors have to be connected as close as possible to the
oscillator pins in order to minimize output distortion and startup stabilization time. The load
capacitance values must be adjusted according to the selected oscillator.
For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 5 pF-to-
25 pF range (typ.), designed for high-frequency applications and selected to meet the
requirements of the crystal or resonator. CL1 and CL2, are usually the same value. The
crystal manufacturer typically specifies a load capacitance that is the series combination of
CL1 and CL2. The PCB and MCU pin capacitances must be included when sizing CL1 and
CL2 (10 pF can be used as a rough estimate of the combined pin and board capacitance).
Refer to the electrical characteristics sections in the datasheet of your product for more
details.

Clocks AN3320
14/29 Doc ID 18267 Rev 2
2.2 LSE OSC clock
The low-speed external clock signal (LSE) can be generated from two possible clock
sources:
●LSE external crystal/ceramic resonator (see Figure 9)
●LSE user external clock (see Figure 8)
1. “LSE crystal/ceramic resonators” figure:
To avoid exceeding the maximum value of CL1 and CL2 (15 pF) it is strongly recommended to use a
resonator with a load capacitance CL ≤ 7 pF. Never use a resonator with a load capacitance of 12.5 pF.
2. “LSE external clock” and “LSE crystal/ceramic resonators” figures:
OSC32_IN and OSC32_OUT pins can be used also as GPIO, but it is recommended not to use them as
both RTC and GPIO pins in the same application.
3. “LSE crystal/ceramic resonators” figure:
The value of REXT depends on the crystal characteristics. A 0 Ωresistor would work but would not be
optimal. To fine tube RSvalue, refer to AN2867 - Oscillator design guide for ST microcontrollers.
2.2.1 External source (LSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
1 MHz. The external clock signal (square, sine or triangle) with a duty cycle of about 50%
has to drive the OSC32_IN pin while the OSC32_OUT pin must be left high impedance (see
Figure 8).
2.2.2 External crystal/ceramic resonator (LSE crystal)
The LSE crystal is a 32.768 kHz low-speed external crystal or ceramic resonator. It has the
advantage of providing a low-power, but highly accurate clock source to the real-time clock
peripheral (RTC) for clock/calendar or other timing functions.
The resonator and the load capacitors have to be connected as close as possible to the
oscillator pins in order to minimize output distortion and startup stabilization time. The load
capacitance values must be adjusted according to the selected oscillator.
Figure 8. LSE external clock Figure 9. LSE crystal/ceramic
resonators
OSC32_OUTOSC32_IN
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2.3 Clock security system (CSS)
The clock security system can be activated by software. In this case, the clock detector is
enabled after the HSE oscillator startup delay, and disabled when this oscillator is stopped.
●If a failure is detected on the HSE oscillator clock, the oscillator is automatically
disabled. A clock failure event is sent to the break input of the TIM1 advanced control
timer and an interrupt is generated to inform the software about the failure (clock
security system interrupt CSSI), allowing the MCU to perform rescue operations. The
CSSI is linked to the Cortex™-M3 NMI (non-maskable interrupt) exception vector.
●If the HSE oscillator is used directly or indirectly as the system clock (indirectly means
that it is used as the PLL input clock, and the PLL clock is used as the system clock), a
detected failure causes a switch of the system clock to the HSI oscillator and the
disabling of the external HSE oscillator. If the HSE oscillator clock (divided or not) is the
clock entry of the PLL used as system clock when the failure occurs, the PLL is
disabled too.
For details, see the STM32F20xxx/21xxx (RM0033) reference manuals available from the
STMicroelectronics website www.st.com.

Boot configuration AN3320
16/29 Doc ID 18267 Rev 2
3 Boot configuration
3.1 Boot mode selection
In the STM32F20xxx/21xxx, three different boot modes can be selected by means of the
BOOT[1:0] pins as shown in Ta bl e 1 .
The values on the BOOT pins are latched on the 4th rising edge of SYSCLK after a reset. It
is up to the user to set the BOOT1 and BOOT0 pins after reset to select the required boot
mode.
The BOOT pins are also resampled when exiting the Standby mode. Consequently, they
must be kept in the required Boot mode configuration in the Standby mode. After this startup
delay has elapsed, the CPU fetches the top-of-stack value from address 0x0000 0000, and
starts code execution from the boot memory starting from 0x0000 0004.
3.2 Boot pin connection
Figure 10 shows the external connection required to select the boot memory of the
STM32F20xxx/21xxx.
Figure 10. Boot mode selection implementation example
1. Resistor values are given only as a typical example.
Table 1. Boot modes
BOOT mode selection pins
Boot mode Aliasing
BOOT1 BOOT0
x 0 Main Flash memory Main Flash memory is selected as boot
space
0 1 System memory System memory is selected as boot
space
1 1 Embedded SRAM Embedded SRAM is selected as boot
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AN3320 Boot configuration
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3.3 Embedded boot loader mode
The Embedded boot loader mode is used to reprogram the Flash memory using one of the
available serial USART1(PA9/PA10), USART3(PB10/11 & PC10/11), CAN2(PB5/13) or USB
OTG FS(PA11/12) in Device mode (DFU: device firmware upgrade).
The USART peripheral operates with the internal 16 MHz oscillator (HSI). The CAN and
USB OTG FS, however, can only function if an external clock (HSE) multiple of 1 MHz
(between 4 and 26 MHz)is present.
This embedded boot loader is located in the System memory and is programmed by ST
during production.
For additional information, refer to AN2606.

Debug management AN3320
18/29 Doc ID 18267 Rev 2
4 Debug management
4.1 Introduction
The Host/Target interface is the hardware equipment that connects the host to the
application board. This interface is made of three components: a hardware debug tool, a
JTAG or SW connector and a cable connecting the host to the debug tool.
Figure 11 shows the connection of the host to the evaluation board STM3220G-EVAL.
Figure 11. Host-to-board connection
4.2 SWJ debug port (serial wire and JTAG)
The STM32F20xxx/21xxx core integrates the serial wire / JTAG debug port (SWJ-DP). It is
an ARM® standard CoreSight™ debug port that combines a JTAG-DP (5-pin) interface and
a SW-DP (2-pin) interface.
●The JTAG debug port (JTAG-DP) provides a 5-pin standard JTAG interface to the AHP-
AP port
●The serial wire debug port (SW-DP) provides a 2-pin (clock + data) interface to the
AHP-AP port
In the SWJ-DP, the two JTAG pins of the SW-DP are multiplexed with some of the five JTAG
pins of the JTAG-DP.
4.3 Pinout and debug port pins
The STM32F20xxx/21xxx MCU is offered in various packages with different numbers of
available pins. As a result, some functionality related to the pin availability may differ from
one package to another.
4.3.1 SWJ debug port pins
Five pins are used as outputs for the SWJ-DP as alternate functions of general-purpose
I/Os (GPIOs). These pins, shown in Ta b l e 2 , are available on all packages.
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AN3320 Debug management
Doc ID 18267 Rev 2 19/29
4.3.2 Flexible SWJ-DP pin assignment
After reset (SYSRESETn or PORESETn), all five pins used for the SWJ-DP are assigned as
dedicated pins immediately usable by the debugger host (note that the trace outputs are not
assigned except if explicitly programmed by the debugger host).
However, some of the JTAG pins shown in Ta b l e 3 can be configured to an alternate function
through the GPIOx_AFRx registers.
Tabl e 3 shows the different possibilities to release some pins.
For more details, see the STM32F20xxx/21xxx (RM0033) reference manual, available from
the STMicroelectronics website www.st.com.
4.3.3 Internal pull-up and pull-down resistors on JTAG pins
The JTAG input pins must not be floating since they are directly connected to flip-flops to
control the debug mode features. Special care must be taken with the SWCLK/TCK pin that
is directly connected to the clock of some of these flip-flops.
Table 2. Debug port pin assignment
SWJ-DP pin name
JTAG debug port SW debug port Pin
assignment
Type Description Type Debug assignment
JTMS/SWDIO I JTAG test mode
selection I/O Serial wire data input/output PA 1 3
JTCK/SWCLK I JTAG test clock I Serial wire clock PA14
JTDI I JTAG test data input - - PA15
JTDO/TRACESWO O JTAG test data output - TRACESWO if async trace
is enabled PB3
JNTRST I JTAG test nReset - - PB4
Table 3. SWJ I/O pin availability
Available Debug ports
SWJ I/O pin assigned
PA13 /
JTMS/
SWDIO
PA14 /
JTCK/
SWCLK
PA15 /
JTDI
PB3 /
JTDO
PB4/
JNTRST
Full SWJ (JTAG-DP + SW-DP) - reset state X X X X X
Full SWJ (JTAG-DP + SW-DP) but without
JNTRST XXXX
JTAG-DP disabled and SW-DP enabled X X
JTAG-DP disabled and SW-DP disabled Released

Debug management AN3320
20/29 Doc ID 18267 Rev 2
To avoid any uncontrolled I/O levels, the STM32F20xxx/21xxx embeds internal pull-up and
pull-down resistors on JTAG input pins:
●JNTRST: Internal pull-up
●JTDI: Internal pull-up
●JTMS/SWDIO: Internal pull-up
●TCK/SWCLK: Internal pull-down
Once a JTAG I/O is released by the user software, the GPIO controller takes control again.
The reset states of the GPIO control registers put the I/Os in the equivalent state:
●JNTRST: Input pull-up
●JTDI: Input pull-up
●JTMS/SWDIO: Input pull-up
●JTCK/SWCLK: Input pull-down
●JTDO: Input floating
The software can then use these I/Os as standard GPIOs.
Note: The JTAG IEEE standard recommends to add pull-up resistors on TDI, TMS and nTRST but
there is no special recommendation for TCK. However, for the STM32F20xxx/21xxx, an
integrated pull-down resistor is used for JTCK.
Having embedded pull-up and pull-down resistors removes the need to add external
resistors.
4.3.4 SWJ debug port connection with standard JTAG connector
Figure 12 shows the connection between the STM32F20xxx/21xxx and a standard JTAG
connector.
Figure 12. JTAG connector implementation
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