S130 SDS v2.0: BLE central connection processor usage patterns

This section describes the processor availability and interrupt processing time for the SoftDevice in a central connection event.

Figure 1. Central connection events

In a central connection event, the pattern of SoftDevice processing activity at interrupt priority level 0 is typically as follows:

First, there is an interrupt (Radio prepare) that sets up and prepares the software and hardware.
Then there is a short interrupt when the radio starts transmitting the first packet in the connection event.
When the transmission is finished, there is Radio processing that switches the radio to reception.
When the reception is finished, there is a Radio processing that processes the received packet, and either including a switch back to transmission and possibly a new reception after that, or the event ends with post processing.
After the radio and post processings in priority level 0, the SoftDevice processes any received data packets, executes any GATT, ATT or SMP operations and generates events to the application as required in priority level 2. The interrupt at this priority level is therefore highly variable based on the stack operations executed.

SoftDevice processing activity in the different priority levels during central connection events is outlined in Table 1. The typical case is seen when receiving GATT write commands writing 20 bytes. The max case can be seen when sending and receiving maximum length packets and at the same time initiating encryption, 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 latency when connected
Parameter	Description	Min	Typical	Max
t_{ISR(0),RadioPrepare}	Processing preparing the radio for a connection event.		96 μs	120 μs
t_{ISR(0),RadioStart}	Processing when starting the connection event.		61 μs	70 μs
t_{ISR(0),RadioProcessing}	Processing after sending or receiving a packet.		80 μs	150 μs
t_{ISR(0),PostProcessing}	Processing at the end of a connection event.		270 μs	730 μs
t_nISR(0)	Distance between connection event interrupts.	30 μs	> 155 μs
t_ISR(2)	Priority level 2 interrupt after a packet is sent or received.		160 μs

From the table, we can calculate a typical processing time for a central connection event where one packet is sent and received to be

t_{ISR(0),RadioPrepare} + t_{ISR(0),RadioStart} + t_{ISR(0),RadioProcessing} + t_{ISR(0),PostProcessing} + 2 * t_ISR(2) = 827 μs

which means that typically more than 99% of the processor time is available to the application when one peripheral link is established and one packet is sent in each direction with a 100 ms connection interval.