U3742BM Datasheet, PDF (12/32 Pages) ATMEL Corporation – UHF ASK/FSK RECEIVER

Polling Mode
Sleep Mode
According to Figure 13 on page 14, the receiver stays in polling mode in a continuous
cycle of three different modes. In sleep mode, the signal processing circuitry is disabled
for the time period T_Sleep while consuming low current of I_S = I_Soff. During the start-up
period, T_Startup, all signal processing circuits are enabled and settled. In the following bit
check mode, the incoming data stream is analyzed bit by bit contra a valid transmitter
signal. If no valid signal is present, the receiver is set back to sleep mode after the
period T_Bitcheck. This period varies check by check as it is a statistical process. An aver-
age value for T_Bitcheck is given in the electrical characteristics. During T_Startup and T_Bitcheck
the current consumption is I_S = I_Son. The average current consumption in polling mode is
dependent on the duty cycle of the active mode and can be calculated as:
I_Spoll
=
-^I-^S----^o--^f-^f---^´-----^T----^S---^l-^e---^e--^p----+-----^I--^S---^o---ⁿ----^´-----⁽--^T----^S---^t--^a--^r--^t-^u---^p----+-----^T---^B----ⁱ-^t-^c--^h---^e--^c---^k---⁾
T_Sleep + T_Startup + T_Bitcheck
During T_Sleep and T_Startup, the receiver is not sensitive to a transmitter signal. To guaran-
tee the reception of a transmitted command, the transmitter must start the telegram with
an adequate preburst. The required length of the preburst is dependent on the polling
parameters T_Sleep, T_Startup, T_Bitcheck and the startup time of a connected microcontroller
(T_{Start,microcontroller}). T_Bitcheck thus depends on the actual bit rate and the number of bits
(N_Bitcheck) to be tested.
The following formula indicates how to calculate the preburst length.
T_Preburst ³ T_Sleep + T_Startup + T_Bitcheck + T_{Start_microcontroller}
The length of period T_Sleep is defined by the 5-bit word Sleep of the OPMODE register,
the extension factor X_Sleep, according to Table 8 on page 21, and the basic clock cycle
T_Clk. It is calculated to be:
T_Sleep = Sleep ´ X_Sleep ´ 1024 ´ T_Clk
In US- and European applications, the maximum value of T_Sleep is about 60 ms if X_Sleep
is set to 1. The time resolution is about 2 ms in that case. The sleep time can be
extended to almost half a second by setting X_Sleep to 8. X_Sleep can be set to 8 by bit
XSleep_Std or by bit XSleep_Temp resulting in a different mode of action as described
below:
XSleep_Std = 1 implies the standard extension factor. The sleep time is always extended.
XSleep_Temp = 1 implies the temporary extension factor. The extended sleep time is used
as long as every bit check is OK. If the bit check fails once, this bit is set back to 0 auto-
matically resulting in a regular sleep time. This functionality can be used to save current
in the presence of a modulated disturber similar to an expected transmitter signal. The
connected microcontroller is rarely activated in that condition. If the disturber disap-
pears, the receiver switches back to regular polling and is again sensitive to appropriate
transmitter signals.
According to Table 7 on page 21, the highest register value of Sleep sets the receiver
into a permanent sleep condition. The receiver remains in that condition until another
value for Sleep is programmed into the OPMODE register. This function is desirable
where several devices share a single data line.
12 U3742BM
4735A–RKE–11/03