UM0470 datasheet(7/37 Pages) STMICROELECTRONICS

7 / 37 page

UM0470

Single wire interface module (SWIM)

7/37

3.2

SWIM entry sequence

After a POR, and as long as the SWIM module is in OFF mode, the SWIM pin is sampled for

entry sequence detection. In order to do this, the internal low speed RC clock is

automatically turned ON after POR and remains forced ON as long as the SWIM is in OFF

mode.

If the register which forces the SWIM module in I/O mode is written before the entry

sequence is finalized, the SWIM module enters I/O mode. Once the SWIM module is

ACTIVE, writing this bit has no influence on communication and the SWIM interface remains

in ACTIVE mode.

If an application uses the SWIM pin as standard I/O, it puts the SWIM interface in I/O mode

in the initialization section of the software code (typically, this is performed just after the

reset). However, even in this case, it is still possible to put the SWIM interface in ACTIVE

mode by forcing the RESET pin to 0 and keep it low for the duration of the SWIM entry

sequence.

As long as the SWIM module is in OFF mode, the SWIM entry sequence is detected at any

moment, during reset or when the application is running.

If both the SWIM pin and the reset pin are multiplexed with I/Os, the way to enter SWIM

ACTIVE state is to power down the MCU device, power up and to maintain the reset until the

end of the SWIM entry sequence.

Figure 4.

SWIM activation timing diagram

SWIM

SWIM Entry Sequence

Communication

(1)

(2)

(3)

(4)

LSI osc ON

HSI osc ON

(5)

SWIM active set

SWIM_CSR(5) set

UM0470 Datasheet(PDF) 7 Page - STMicroelectronics