CC2550_13 Datasheet, PDF (26/61 Pages) Texas Instruments – Low-Cost Low-Power 2.4 GHz RF Transmitter

CC2550
sequences must be followed: Automatic
power-on reset (POR) or manual reset.
16.1.1 Automatic POR
A power-on reset circuit is included in the
CC2550. The minimum requirements stated in
Section 4.7 must be followed for the power-on
reset to function properly. The internal power-
up sequence is completed when CHIP_RDYn
goes low. CHIP_RDYn is observed on the SO
pin after CSn is pulled low. See Section 10.1
for more details on CHIP_RDYn.
When the CC2550 reset is completed the chip
will be in the IDLE state and the crystal
oscillator will be running. If the chip has had
sufficient time for the crystal oscillator to
stabilize after the power-on-reset, the SO pin
will go low immediately after taking CSn low. If
CSn is taken low before reset is completed the
SO pin will first go high, indicating that the
crystal oscillator is not stabilized, before going
low as shown in Figure 14.
Figure 14: Power-On Reset
16.1.2 Manual Reset
The other global reset possibility on CC2550 is
the SRES command strobe. By issuing this
strobe, all internal registers and states are set
to the default, IDLE state. The manual power-
up sequence is as follows (see Figure 15):
• Strobe CSn low / high.
• Hold CSn high for at least 40 µs relative to
pulling CSn low
• Pull CSn low and wait for SO to go low
(CHIP_RDYn).
• Issue the SRES strobe on the SI line.
• When SO goes low again, reset is
complete and the chip is in the IDLE state.
XOSC and voltage regulator switched on
40 us
CSn
SO
XOSC Stable
SI
SRES
Figure 15: Power-On Reset with SRES
Note that the above reset procedure is only
required just after the power supply is first
turned on. If the user wants to reset the
CC2550 after this, it is only necessary to issue
an SRES command strobe.
16.2 Crystal Control
The crystal oscillator is automatically turned on
when CSn goes low. It will be turned off if the
SXOFF or SPWD command strobes are issued;
the state machine then goes to XOFF or
SLEEP respectively. This can only be done
from the IDLE state. The XOSC will be turned
off when CSn is released (goes high). The
XOSC will be automatically turned on again
when CSn goes low. The state machine will
then go to the IDLE state. The SO pin on the
SPI interface must be pulled low before the
SPI interface is ready to be used; as described
in Section 10.1 on page 15.
Crystal oscillator start-up time depends on
crystal ESR and load capacitances. The
electrical specification for the crystal oscillator
can be found in Section 4.3 on page 7.
16.3 Voltage Regulator Control
The voltage regulator to the digital core is
controlled by the radio controller. When the
chip enters the SLEEP state, which is the state
with the lowest current consumption, the
voltage regulator is disabled. This occurs after
CSn is released when a SPWD command
strobe has been sent on the SPI interface. The
chip is now in the SLEEP state. Setting CSn
low again will turn on the regulator and crystal
oscillator and make the chip enter the IDLE
state.
All CC2550 register values (with the exception
of the MCSM0.PO_TIMEOUT field) are lost in
SWRS039B
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