System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
3-5
3.4.1.2
Modem Reset
The modem active low reset input M_RST is driven onboard the SiP from MCU GPIO pin PTD3. In the
interest of lowest power, there is no pull-up resistor on input M_RST. Output PTD3 has a software
controlled pull-up resistor, however, it would typically not be used because the modem can be held in
hardware reset by the MCU for extended periods of time for lowest power applications.
From a reset condition, the MCU initiates PTD3 as a high-impedance input with the internal pull-up
disabled. As a result, the modem reset input will be floating and the modem will not be held in reset. As
part of the MCU initialization, PTD3 must be programmed as an output and then driven low to reset the
modem. The M_RST input is asynchronous and needs to be held low for only a short period.
In the reset condition, the modem is totally powered down and no clocks are available. After M_RST is
released, the modem will power up, initialize, and go to its idle condition within 10 - 25 milliseconds, and
in turn, this causes an ATTN interrupt request and allows CLKO to start toggling at 32.768+ kHz (both of
which are default conditions). The ATTN hardware interrupt request is normally caused by asserting
modem signal ATTN, however, coming out of reset the ATTN status bit is set and the ATTN interrupt
request mask is set.
Once the interrupt request is seen by the MCU, the MCU can assume the modem is alive and ready for
programming via the SPI bus.
Modem reset operation and control is detailed in
Chapter 9, “Modem Miscellaneous Functions”
.
3.4.2
Modem Interrupt Request to MCU
The modem interrupt request M_IRQ is an active low open drain output that is asserted when an interrupt
request is pending. The signal is released to high by reading the modem IRQ_Status register via a SPI
transaction. M_IRQ has a programmable pull-up resistor and it also can be programmed for drive strength.
M_IRQ is covered in detail in
Section 8.1.2, “Output Pin IRQ”
.
The modem interrupt request is tied to the MCU external interrupt input IRQ. The MCU IRQ must be
programmed as an external interrupt and can be enabled for edge-only or edge-and-level events. The MCU
IRQ also has a programmable pull-up. IRQ is covered in detail in
Section 8.1, “Modem Interrupts”
In the MC1321x the IRQ must be programmed for falling edge or edge/low-level sensitivity. Also, the
MCU pullup should be selected as a pullup is required for the open drain modem output. On the modem
side, the maximum drive strength is suggested as this will give fastest performance for the interrupt fall
time.
3.4.3
SPI Command Channel
The MCU SPI Module is dedicated to the modem SPI interface. The modem is a slave only and the MCU
SPI must be programmed and used as a master only. Further, the SPI performance is limited by the modem
constraints of 8 MHz maximum SPI clock frequency, and use of the MCU SPI must be programmed to
meet the modem SPI protocol. The SPI bus connections are:
•
MCU PTE4/MOSI1 output drives modem MOSI.
•
Modem MISO output drives MCU PTE3/MISO1.
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