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AN4655

HW methods

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HW methods

If addition of native HW peripherals is not acceptable (e.g. by adding external components

like a dedicated communication coprocessor unit), then the limited number of peripherals

can be compensated by the following basic methods:

1.

Use internal remapping option of dedicated peripheral instances, to change the inputs

and outputs available at different ports, thus establishing a temporary connection of the

microcontroller with a different channel (the user can find informations about alternate

function capability in the pin-out description of datasheets, for each product and its

implemented peripherals).

2.

Multiplexing communication channels into a single communication port while using

multiplexing logic.

3.

Using SWO feature.

The first two methods provide service on selected channel exclusively in dedicated time

slots, and can be used only when contemporary operation and permanent monitoring of

altered channels is not strictly required, as it happens, for example, in:

•

service channels;

•

occasional communication with remote devices;

•

monitoring of sensors with slow data value change;

•

channels used for periodical backup.

These methods are suitable for master communications when any slave activity is expected

upon master request, or when some communication with slave can be missed and can be

easily back re-synchronized when master reconnects back to the channel to be serviced

(listened and talked to). It could be used at slave side, too, when some additional control

signals are provided between slave and master to inform the slave that connection is

established (e.g. chip select or channel ready/busy signalizing when slave can/can’t operate

with the channel).

Anyway, user should take care about inactive state at GPIOs associated with a peripheral

instance bus signals when alternate function is remapped or multiplexed to another channel.

A proper setting ensures emulation of idle condition and prevents any stuck at channels not

currently selected or just not used for communication. This can be achieved with a correct

configuration of GPIO port registers, and partially by adding external components (pull

up/pull down resistors). Note that GPIO logic commonly overtakes control over the

associated pins when alternate functionality is removed (remapped) from the port and user

has to prevent any bus false state detection on such temporarily frozen channels.

For multiplexing, user can use specific control of I/O switches at dedicated groups of routing

interface if it is available for the product. User must ensure that a proper combination (pair)

of switches within a single group be closed in time to let the signal pass through the desired

path. User has to dedicate additional IOs for such switching, so they are lost for other

purposes, and there is additional PCB routing.

The principle is shown in Figure 1 and Figure 2.