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ST7285C Datasheet(HTML) 48 Page - STMicroelectronics

Part No. ST7285C
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com

ST7285C Datasheet(HTML) 48 Page - STMicroelectronics

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The Slave select (SS) pin receives an active-low
signal which is generated by the Master device, in
order to enable Slave devices to accept data.
To ensure that data will be accepted by a Slave
device, the SS line must be at a logic lowlevel prior
to the occurence of SCK (system clock), and must
remain low until after the last (eighth) SCK cycle.
Figure 28 illustrates the relationship between SCK
and the data for two different level combinations of
CPHA, when SS is pulled low. These are :
– a) CPHA = 1 or 0, the first bit of data is applied
to the MISO line for transfer, and,
– b) when CPHA = 0 the Slave device is prevented
from writing to its data register. For further infor-
mation on the effect the SS input and the CPHA
have on the I/O data register, refer to the WCL
status flag in the ”Serial Peripheral Status Regis-
ter description”. A logic high level on theSS sig-
nal forces the MISO (Master In Slave Out) line to
the high-impedance state. Also, SCK and the
MOSI (Master Out Slave In) line are ignored by a
Slave device when its SS signal is at a logic high
When a device is a Master, it constantly monitors
its SS signal input for the presence of a logic low
level. The Master device will become a Slave de-
vice any time its SS signal input is detected as be-
ing at a logic low level. This ensures that only one
Master controls theSS line.
When the SS line is detected as being at a logic
low level, the Master clears the MSTR control bit
(Serial Peripheral Control Register). Also, control
bit SPE in the Serial Peripheral Control Register is
cleared, causing the Serial Peripheral Interface
(SPI) to be disabled (SPI alternate function pins
become inputs). The MODF flag bit in the Serial
Peripheral Status Register is also set to indicate to
the Master device that another device is attempt-
ing to become a Master. Two devices attempting
to be outputs are normally the result of a software
error. However, the user system can be configured
in such manner as to contain a default Master
which would automatically ”take-over” and restart
the system. Serial Clock (SCK)
The Serial Clock is used to synchronize the move-
ment of data both in and out of the device via its
MOSI and MISO pins. The Master and Slave de-
vices are capable of exchanging a data byte of in-
formation during a sequence of eight clock pulses.
Since the SCK is generated by the Master device,
the SCK line becomes an input on all Slave devic-
es and synchronizes Slave data transfer. The type
of clock and its relationship to data are controlled
by the CPOL and CPHA bits in the Serial Peripher-
al Control Register.
The Master device generates the SCK through a
circuit driven by the internal processor clock. Two
bits (SPR0 and SPR1) in the Serial Peripheral
Control Register of the Master device select the
clock rate. The Master device uses the SCK to
latch incoming Slave device data on the MISO line
and shifts out data to the Slave device on the
MOSI line. Both Master and Slave devices must
be operated in the same timing mode as defined
by the CPOL and CPHA bits in the Serial Peripher-
al Control Register.
In the Slave device, SPR0 and SPR1 have no ef-
fect on the operation of the Serial Peripheral Inter-

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