U3745BM Datasheet, PDF (11/29 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

Polling Mode
Sleep Mode
4663A–RKE–06/03
U3745BM
• USA Applications
(f_XTO = 4.90625 MHz, MODE = L, T_Clk = 2.0383 µs)
• Europe Applications
(f_XTO = 6.76438 MHz, MODE = H, T_Clk = 2.0697 µs)
• Other applications
(T_Clk is dependent on f_XTO and on the logical state of pin MODE. The electrical
characteristic is given as a function of T_Clk).
The clock cycle of some function blocks depends on the selected baud rate range
(BR_Range) which is defined in the OPMODE register. This clock cycle T_XClk is defined
by the following formulas for further reference:
BR_Range =
BR_Range0:
BR_Range1:
BR_Range2:
BR_Range3:
T_XClk = 8 ´ T_Clk
T_XClk = 4 ´ T_Clk
T_XClk = 2 ´ T_Clk
T_XClk = 1 ´ T_Clk
According to Figure 3, 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 average value for
T_Bitcheck is given in the section “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,µC). 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_mC}
The length of period T_Sleep is defined by the 5-bit word Sleep of the OPMODE register,
the extension factor XSleep, according to table 10, 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 XSleep
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 XSleep to 8. XSleep 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.
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