S2-LP datasheet(64/90 Pages) STMICROELECTRONICS

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Link layer protocol

S2-LP

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DocID029944 Rev 1

8.2.1

Low duty cycle mode

The S2-LP provides an embedded low duty cycle mode (LDC), that allows reducing the

average power consumption during receive operations and to build a synchronized start

network where both transmitter and receiver can go in low power mode periodically to

reduce average power consumption.

The LDC mode is controlled essentially by the LDC timer, which periodically wakes up the

S2-LP to perform a transmission or a reception.

In reception mode, it is also relevant to set up the RX timer in order to minimize the amount

of time the S2-LP waits for a packet. The RX timer defines the RX windows within a valid

SYNC word should be detected.

As shown in Figure 13: "Common RX operation", a common receiver usually stays in RX

state for long time waiting of the TX packet.

Figure 13: Common RX operation

Using the LDC mode, the S2-LP will wake up periodically saving a lot of power.

Figure 14: LDC RX operation

If synchronization between transmitter and receiver is required, a programmable timer

value can be reloaded at SYNC word detection by the receiver or by SPI command.

The timer used to wait for the wakeup (TWU) is clocked by the signal generated by the RCO

circuit (or by an external clock from a GPIO pin), and is programmable with the registers

LDC_PRESCALER and LDC_COUNTER. The internal RC oscillator used by the LDC timer

must be calibrated just before the LDC mode is used.

After the wake-up signaling from its internal timer, the S2-LP will switch to RX (TX) state

and an interrupt request will be issued (if enabled and not masked). In order to allow for

analog circuits settling, an idle time TIDLE should be allowed before effective operation: the

effective reception starting time is related to the synchronization with the sender. The idle

time could result longer than the minimum required to get RX circuits settling, and this

cause power wasting. In order to minimize the TIDLE, S2-LP supports the runtime phasing

of the internal wake-up timer, as follows:



For both RX and TX devices, the value of the wake-up timer can be reloaded during

runtime using the LDC_RELOAD command with the values written in the

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