S130 SDS v2.0: BLE advertiser (broadcaster) processor usage patterns

This section describes the processor availability and interrupt processing time for the SoftDevice when the advertiser (broadcaster) role is running.

Figure 1. Advertising events

When advertising, the pattern of SoftDevice processing activity for each advertising interval, at interrupt priority level 0 is as follows:

First, there is an interrupt (Radio prepare) that sets up and prepares the software and hardware for this advertising event.
Then there is a short interrupt when the radio starts sending the first advertising packet.
Depending on the type of advertising, there may be one or more instances of Radio processing (including processing in priority level 2) and further receptions/transmissions.
Finally, advertising ends with Post processing at interrupt priority level 0 and some interrupt priority level 2 activity.

SoftDevice processing activity in the different priority levels when advertising is outlined in Table 1. The typical case is seen when advertising without using a whitelist and without receiving scan or connect requests. The max case can be seen when advertising with a full whitelist, receiving scan and connect requests while having a maximum number of connections and utilizing the Radio Timeslot API and Flash memory API at the same time.

Table 1. Processor usage when advertising
Parameter	Description	Min	Typical	Max
t_{ISR(0),RadioPrepare}	Processing preparing the radio for advertising.		76 μs	130 μs
t_{ISR(0),RadioStart}	Processing when starting the advertising.		36 μs	40 μs
t_{ISR(0),RadioProcessing}	Processing after sending/receiving a packet.		50 μs	80 μs
t_{ISR(0),PostProcessing}	Processing at the end of an advertising event.		200 μs	700 μs
t_nISR(0)	Distance between interrupts during advertising.	40 μs	>150 μs
t_ISR(2)	Priority level 2 interrupt at the end of an advertising event.		270 μs

From the table, we can calculate a typical processing time for one advertisement event sending three advertisement packets to be

t_{ISR(0),RadioPrepare} + t_{ISR(0),RadioStart} + 2 * t_{ISR(0),RadioProcessing} + t_{ISR(0),PostProcessing} + t_ISR(2) = 682 μs

which means that typically more than 99% of the processor time is available to the application when advertising with a 100 ms interval.