LEA-6 / NEO-6 / MAX-6 - Hardware Integration Manual
UBX-14054794
Production Information
Appendix
Page 79 of 85
device is not selected (i.e. its chip select is not activated), its data output enters a high-impedance state (hi-Z) and
does not interfere with SPI bus activities.
The data output MISO functions as the data return signal from the slave to the master.
Figure 69 shows a typical block diagram for an SPI master with several slaves. Here, the SCK and MOSI data lines
are shared by all of the slaves. Also the MISO data lines are linked together and led back to the master. Only the
chip selects are separately brought to each SPI device.
SPI Master
SPI Slave
0
C
hi
p
Se
le
ct
MOSI
SCK
Data Input
MISO
SPI Slave
1
SPI Slave
2
SCS
0
SCS
1
SCS
2
C
lo
ck
D
at
a
O
ut
pu
t
MOSI
MOSI
MOSI
SS_N
SS_N
SS_N
SCK
SCK
SCK
MISO
MISO
MISO
Figure 69: Master with independent slaves
SPI allows multiple microcontrollers to be linked together. These can be configured according to single or
multiple master protocols. In the first variant the microcontroller(s) designated as slave(s) behave like a normal
peripheral device. The second variant allows for several masters and allows each microprocessor the possibility to
take the role of master and to address another microprocessor. In this case one microcontroller must
permanently provide the clock signal.
There are two SPI system errors. The first occurs if several SPI devices want to become master at the same time.
The other is a collision error that occurs for example when SPI devices work with different polarities.
Systems involving multiple microcontrollers are beyond the scope of this document.
Cascading slave peripherals is not supported.
Four I/O pin signals are associated with SPI transfers: the SCK, the MISO data line, the MOSI data line, and the
active low SCS/SS_N pin. In the unselected state the MISO lines are hi-Z and therefore inactive. The master
decides with which peripheral device it wants to communicate. The clock line SCK provides synchronization for
data communication and is brought to the device whether or not it is selected.
