DocID027290 Rev 1

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AN4635

Reliability and communication quality

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5

Reliability and communication quality

5.1

Noise and frequency shift

Line noise should cause no issues at the low communication speed typical for low power

applications. In some cases, random noise can be mitigated by adding a weak pull-up. This

is of course at expense of additional energy. For example the internal pull-up (no extra

parts) increases power consumption by approximately 30 µA during transmission in the

example described in Section 7.

Framing errors occur as result of incorrect base frequency (speed setting) at one of the

communication participants. The LSE clock source, which is best suited for low power

applications, is very accurate. Only if LPUART peripheral clock is derived from MSI clock

then the framing errors are very likely. The LPUART also cannot auto detect the

communication speed of its counterpart, relying only on the precise speed settings with

framing reliability.

5.2

Dropped bytes

The main means of achieving lower power consumption is to tune down core frequency and

use suspension to low power modes. This imposes a challenge of waking up and

responding to peripheral events.

In transmission case this may cause delay between bytes sent, leading to prolonged period

of activity and thus impact on system efficiency, but no data loss.

The true problem is data reception. In polling or interrupt driven mode, if the received data in

the LPUART data register is not read in time i.e. before a new data is received, an overrun

error occurs and any data received during overrun is lost.

To avoid missing data in reception, the following guidelines can be provided:

•

ensure a correct clock speed (not below a certain minimum) for the core;

•

boost the LPUART interrupts priorities to the highest possible level in the application to

make them basically uninterruptible;

•

use the DMA for data transfers.