NXP Semiconductors freescale semiconductor MC13211, Справочное руководство

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Modem Miscellaneous Functions
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 9-3
9.2.5 GPIO1 and GPIO2 as Stream Mode Status Indicators
To support easier and quicker status indication for the MCU, GPIO1 and GPIO2 can be programmed for
special alternative functionality. If the gpio_alt_en bit of Control_C Register 09 is set to 1 then:
1. GPIO1 becomes an “Out of Idle” indicator (active high) - GPIO1 will always reflect the status of
the internal state machine. If the MC1321x is in a TX or RX or CCA/ED sequence, the GPIO1 will
be high. Once the sequence ends, the GPIO1 returns to a low state and shows that the transceiver
has returned to Idle. In Doze or Hibernate Mode GPIO1 stays low.
2. GPIO2 becomes a “Valid CRC” or “Valid CCA Result” indicator (active high) -
a) For a RX sequence, the GPIO2 will show the CRC is valid once the RX operation is complete
and GPIO1 goes to low to indicate a return to Idle condition. The transition of GPIO1 from high
to low latches the CRC result on GPIO2 and that status will not change until the next
transceiver sequence. GPIO2 will not be valid if an error condition such as a PLL out-of-lock
condition occurs.
b) For a CCA sequence, the GPIO2 will show if the CCA is valid once the CCA operation is
complete and GPIO1 goes to low to indicate a return to Idle condition. The transition of GPIO1
from high to low latches the CCA result on GPIO2 and that status will not change until the next
transceiver sequence. GPIO2 will not be valid if an error condition such as a PLL out-of-lock
condition occurs.
These two signals can be used by the MCU to monitor transceiver status without interrogating the onboard
status registers.
NOTE
GPIO1 and GPIO2 should also be programmed as outputs for this function.
9.2.6 GPIO in Off, Hibernate, and Doze Modes
The GPIO can affect excess leakage current during low power modes. The best practice is to tie unused
GPIO pins to ground through a resistor. In Hibernate and Doze, the state of the signals are retained and a
GPIO programmed as an output will remain an output. Programming outputs low uses the absolute lowest
power.
The Off condition can present a problem. In the Off Mode the GPIO resort to inputs and can float if not
tied down which can cause excessive leakage current (the reason for the off condition is lowest current).
This is why the best practice is to tie unused inputs low. Also, if a GPIO is used to interface to another
device, a pullup or pull-down resistor should be in place to hold the GPIO pin to a known state.
9.3 Crystal Oscillator
The crystal oscillator for the MC1321x uses the following external pins:
1. XTAL1 - reference oscillator input.
2. XTAL2 - reference oscillator output. Note that this pin should not be loaded to be used as a
reference source or to measure frequency; instead use CLKO to measure or supply 16 MHz.
The external crystal circuit is shown in Figure 9-1 .