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Slave Port Interface
NOTE
The MMA955xL platform includes both interfaces. The selection of the
operating mode between I 2 C and SPI is initialized at startup. If SSB
(RGPIO3) is found to be low during the boot process, SPI mode will be
Immediately after a device reset, the state of pin number 8 (RGPIO3 / SDA1
/ SSB functions) is used to select the slave port interface mode. This implies
important rules in the way the host controller, or more generally, the
complete system should be handling this pin.
First of all, whenever a reset occurs on MMA955xL, RGPIO3 pin level shall
be consistent with the interface mode of operation. This is particularly
important if this pin is driven from external devices. If RGPIO3 level does
not match the current mode of operation, an alternate mode is selected and
communication with the host is lost.
If I 2 C mode is used, a good practice is to tie RGPIO3 to a pull-up resistor so
that it defaults to high level. Note that such a connection exists when the
Master I 2 C interface is used (SDA1 function for pin 8).
When using SPI mode for the slave interface, the situation is more complex
as the same pin plays two roles: SSB and mode selection. Moreover, after a
SPI read or write operation, the SSB line returns to high level.
Consequently, if the host sends a command to the MMA955xL that induces
a subsequent reset immediately after the write transaction, it forces the SSB
line to low level. This means that SPI mode is still selected after reset. The
duration for the SSB line to be kept low typically depends on the latency
between the write transaction and the execution of the reset command. Such
latency can be significant for the MMA9553L pedometer firmware as the
Command Interpreter and Scheduler applications are running at 30Hz which
gives a 33ms typical latency.
The rule obviously applies also when a hardware reset is issued by the host
through MMA955xL pin 3 (RESETB active low). Again the host has to
drive the SSB line low prior to releasing the hardware reset line to high
level, which immediately triggers the MMA955xL reset and boot sequence.
Keeping the SSB line low during 1 ms duration after RESETB is released is
enough for the MMA955xL slave device to re-boot into SPI mode.
MMA955xL Intelligent, Motion-Sensing Platform Hardware Reference Manual, Rev. 1.0
116
Freescale Semiconductor, Inc.
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