S130 SDS v2.0: BLE scanner and initiator processor usage patterns

This section describes the processor availability and interrupt processing time for the SoftDevice when the scanner or initator role is running.

Figure 1. Scanning or initiating

When scanning or initiating, the pattern of SoftDevice processing activity 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 scanner or initiator event.
Then there is a short interrupt when the radio starts listening for advertisement packets.
During scanning, there will be zero or more instances of Radio processing, depending upon whether the active role is a scanner or an initator, whether scanning is passive or active, whether advertising packets are received or not and upon the type of the received advertising packets. Such Radio processing may be followed by the SoftDevice processing at interrupt priority level 2.
When the event ends (either by timeout, or if the initator receives a connectable advertisement packet it accepts), the SoftDevice does some Post processing, which may be followed by processing at interrupt priority level 2.

SoftDevice processing activity in the different priority levels when scanning or initiating is outlined in Table 1. The typical case is seen when scanning or initiating without using a whitelist and without sending scan or connect requests. The max case can be seen when scanning or initiating with a full whitelist, sending scan or 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 for scanning or initiating
Parameter	Description	Min	Typical	Max
t_{ISR(0),RadioPrepare}	Processing preparing the radio for scanning or initiating.		55 μs	128 μs
t_{ISR(0),RadioStart}	Processing when starting the scan or initiation.		60 μs	80 μs
t_{ISR(0),RadioProcessing}	Processing after sending/receiving packet.		82 μs	145 μs
t_{ISR(0),PostProcessing}	Processing at the end of a scanner or initiator event.		186 μs	620 μs
t_nISR(0)	Distance between interrupts during scanning.	30 μs	>1.5 ms
t_ISR(2)	Priority level 2 interrupt at the end of a scanner or initiator event.		250 μs

From the table, we can calculate a typical processing time for one scan event receiving one advertisement packet to be

t_{ISR(0),RadioPrepare} + t_{ISR(0),RadioStart} + t_{ISR(0),RadioProcessing} + t_{ISR(0),PostProcessing} + t_ISR(2) = 633 μs

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