AN3342 datasheet(17/39 Pages) STMICROELECTRONICS

17 / 39 page

AN3342

Recommendations

Doc ID 18461 Rev 3

17/39

6.5

Other signals

When designing an application, the following areas should be closely studied to improve

EMC performances:

●

Noisy signals (clock)

●

Sensitive signals (high impedance)

●

Signals for which a temporary disturbance affects operation of the application

permanently, for example, interrupts and handshaking strobe signals (but not LED

commands).

A surrounding VSS trace for such signals increases EMC performances, as does a shorter

length or absence of noisy and sensitive traces (crosstalk effect).

For digital signals, the best possible electrical margin must be reached for the two logical

states. Slow Schmitt triggers are recommended for eliminating parasitic states.

6.6

Unused I/Os and features

Microcontrollers are designed for a variety of applications, where often a particular

application does not use 100 % of the microcontroller resources.

To avoid unnecessary power consumption (especially important for battery powered

applications) and also to improve EMC performance, unused clocks, counters, or I/Os,

should not be left free, I/Os should be forced externally (pull-up or pull-down to the unused

I/O pins), and unused functions should be ‘frozen’ or disabled.

Alternatively, unused I/Os can be programmed as push-pull ‘low’ to keep them at a defined

level without using external components. However in this case, the I/O is not driven during

the power up phase, until the I/O is configured. This can add a little extra power

consumption, and may be undesirable in very power sensitive applications.

The unused PXS_RX pins should follow the same rule and can be driven to VSS by resetting

the PXS_RXINSR register.

6.6.1

20-pin package

A special care of I/O configuration must be taken with STM8TL5xFx devices. The port A, B

and D I/Os are not all configured by default as it is the case on 28-pin packages.

The user code must configure PA6, PA7, PD2, PD3, PD7 as output push pull low level. It is

recommended to program all the unused I/Os to push-pull 'low level' even the ones not

present in the device. While accessing to the GPIO registers, it is strongly recommended to

mask the unused bit in order not to change their configuration or state.

Concerning the PXS_RX: PXS_RXENRH and PXS_RXINSRH (and more generally all the

MSByte of the register related to PxS_RX and PXS_TX) must be kept clear.

AN3342 Datasheet(PDF) 17 Page - STMicroelectronics

AN3342 Datasheet(HTML) 17 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Datasheet Download

Link URL