MC1321x Serial Peripheral Interface (SPI)
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
4-3
4.4
Modem SPI Overview
Control of the modem and data transfers between the modem and CPU are accomplished by means of the
4-wire SPI. The modem SPI port is a fully static design that requires no additional clocks besides SPICLK
for accessing internal registers, although Packet RAM accesses do require the modem reference oscillator to be
running. This allows for lower power when the SPI must stay alive for data retention while the rest of the device is
in power-down.
Although the normal SPI protocol is based on 8-bit transfers, the modem imposes a higher level transaction
protocol that is based on multiple 8-bit transfers per transaction. In its simplest form, a singular SPI read
or write transaction consists of an 8-bit header transfer followed by two 8-bit data transfers. The header
denotes access type and register address. The following bytes are read or write data. The SPI also supports
recursive ‘data burst’ transactions in which additional data transfers can occur. The Recursive Mode is primarily
intended for Packet RAM access and fast configuration of the MC1321x. Partial word accesses are not supported.
All modem SPI accessible registers are configured with 16-bit data width. The address range is 6 bits
which allows for 64 locations although not all are implemented. Internal data RAMs are accessed as
dedicated addresses within the register address field.
An additional feature is a software reset capability where a write to Address 00 will accomplish most of
the equivalency of a hardware reset.
4.5
Modem SPI Basic Operation
The modem operates as a SPI slave only. The microcontroller supplies the interface clock and acts as SPI
master.
4.5.1
Modem SPI Pin Definition
The modem signals of CE, SPICLK, MOSI, and MISO are defined in the following paragraphs.
4.5.1.1
Chip Enable (CE)
A transaction on the SPI port is framed by the active low Chip Enable (CE) input signal which is driven
by the MCU. A transaction is a minimum of 3 SPI byte bursts and can extend to a greater number of bursts.
4.5.1.2
SPI Clock (SPICLK)
The host drives the SPI Clock (SPICLK) input to the modem. Data is clocked into the master or slave on
the leading (rising) edge of the return-to-zero SPICLK and data out changes state on the trailing (falling)
edge of SPICLK.
NOTE
For theHCS08 microcontroller, the SPI clock format is the clock phase
control bit CPHA = 0 and the clock polarity control bit CPOL = 0.
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