UM0470 datasheet(5/37 Pages) STMICROELECTRONICS

5 / 37 page

UM0470

Communication layer

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Communication layer

The SWIM module is a single wire interface based on asynchronous, high sink (8 mA),

open-drain, bidirectional communication.

While the CPU is running, the SWIM module allows non-intrusive read/write accesses to be

performed on-the-fly to the RAM and peripheral registers, for debug purposes.

In addition, while the CPU is stalled, the SWIM module allows read/write accesses to be

performed to any other part of the MCU’s memory space (Data EEPROM and program

memory).

CPU registers (A, X, Y, CC, SP) can also be accessed. These registers are mapped in

memory and can be accessed in the same way as other memory addresses.

●

When the system is in HALT, WFI or readout protection mode, the NO_ACCESS flag in

the SWIM_CSR register is set. In this case, it is forbidden to perform any accesses

because parts of the device may not be clocked and a read access could return

garbage or a write access might not succeed.

The SWIM module can perform a MCU device software reset.

The SWIM pin can also be used by the MCU target application as a standard I/O port with

some restrictions if you also want to use it for debug. The safest way is to provide a strap

option on the application PCB.

Figure 2.

SWIM pin external connections

STM8

Application I/O

SWIM interface for tools

Jumper selection for

debug purposes

SWIM pin

UM0470 Datasheet(PDF) 5 Page - STMicroelectronics