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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Ω

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