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Reducing device power consumption (Part 3)

Posted: 02 Sep 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Packet classification? power consumption? network standby? Communications Processor? autorespond proxy?

Part 1 of this series tackles the cyclical states of embedded systems. Part 2 delves into the different techniques for low power network standby.

Consider the following real-world example of how low power network standby and advanced power management features can minimise overall system energy consumption:

In this example, only the power of the embedded processor that includes I/O is considered, rather than overall system power. The power in network standby is the main concern C not the significantly higher power required when performing functions such as printing or recording a TV programme.

The system under consideration has the following properties, based on a system incorporating a product such as the QorIQ P1022:
???1.8 seconds to wake to full operation, including voltage ramp-up and software boot. This assumes a mid-range combination of the 1.07s X-Windows and 1.9s Android wake times [9] and the Windowÿ8 wake from S3 state of 2.0 seconds [8].
???1ms per packet to process a packet (based on the assumption of a host with a maximum processing rate of up to 1 million packets per second).
???300mW SoC power dissipation when in network standby mode at room temperature (25C junction temperature).
???5W SoC power dissipation when active and running typical code, 65C junction temperature.

In an otherwise idle network, packets, regardless of whether they require response or not, are assumed to arrive every 80ms. This is equal to the 12.5 packets per second that S Gobrielÿet. al measured [2].

Packets that require response arrive on average every 3s, but only require response every 60s (these values are protocol and network-dependent, but represent realistic assumptions).

Packets that cannot be responded to by an autorespond proxy arrive relatively rarely, less often than once every 60 seconds, and in the context of an autorespond proxy, such a distinction can be ignored from an average system power consumption perspective.

In a legacy system that does not implement either packet classification or packet accumulation, the system would never be able to spend any meaningful time in a deep sleep mode. This is because the time to wake (1.8s) is significantly longer than the time between arrival of packets (80ms). On average, each time the system enters deep sleep, it would be in deep sleep for only 40ms (half the average packet arrival rate), and it would be awake for at least 1.8s. Therefore the average power required to maintain network standby would be equal to the max power, namely 5W.

In a system implementing packet classification but not packet accumulation (such as the MPC8536E Communications Processor [7]), the system would wake on every interesting packet (on average every 3s), and would be awake for the time to boot plus the time to process one packet. This is a period of (1.8s + 1ms) every 3s. Therefore the average power required to maintain network standby with packet classification would be

Pave = tactive /ttotal x Pactive + (ttotaltactive)/ttotal x Pnsÿ(Eq. 1)

Where:
Pave = average power in a given time period ttotal.
Pactive = power consumption when in active mode.
Pns = power consumption when in network standby mode.
tactive = time during ttotal when in active mode (not in network standby), or in the process of waking into this mode.
ttotal = total length of the time period under consideration.

UsingÿEq. 1, the power dissipated in a system with packet classification but not packet accumulation is:

(1.8s + 1ms)/3s x 5W + (3s1.8s1ms)/3s x 300mW
= .60 x 5W + .40 x 300mW
= 3.12W.

This is summarised in the table below.

Table: Average power consumption for various network standby techniques.

In a system implementing both packet classification and packet accumulation (such as [10]), the system would wake every 60s in order to respond to packets before the worst-case 60 second timeout. On average, 20 packets requiring response would have arrived in that time. In this system, tactive then reduces to 1.8s + 20 x 1ms, which is the time to wake up plus the time when in active.

Therefore, using Eq. 1 once again, the average power required to maintain network standby with both packet classification and packet accumulation would be:

(1.8s + 20 x 1ms)/60s x 5W + (60s1.8s C 20 x 1ms)/60s x 300mW
= .03 x 5W + .97 x 300mW
= 0.44W.

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