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ST7285C Datasheet(HTML) 51 Page - STMicroelectronics

Part No. ST7285C
Description  8-BIT MCU FOR RDS WITH 48K ROM, 3K RAM, ADC, TWO TIMERS, TWO SPIs, I2C AND SCI INTERFACES
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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ST7285C Datasheet(HTML) 51 Page - STMicroelectronics

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ST7285C
SERIAL PERIPHERAL INTERFACE(Cont’d)
4.4.8 Serial Peripheral Status Register (SPSR)
Address: see Memory Map
Read Only
Reset Value: 00h
The status flags which generate a Serial Peripher-
al Interface (SPI) interrupt may be blocked by the
SPIE control bit in the Serial Peripheral Control
Register. The WCOL bit does not cause an inter-
rupt. The Serial Peripheral Status register bits are
defined as follows:
Bit-7 = SPIF Serial Peripheral Data Transfer Flag
The Serial Peripheral Data Transfer Flag bit noti-
fies the user that a data transfer between the de-
vice and an external device has been completed.
With the completion of the data transfer, SPIF is
set, and if SPIE is set, a Serial Peripheral Interrupt
is generated. During the clock cycle SPIF is being
set, a copy of the received data byte in the shift
register is moved to a buffer. When the data regis-
ter is read, it is the buffer that is read. In the event
of an overrun condition, when the Master device
has sent several bytes of data and the Slave de-
vice has not responded to the first SPIF, only the
first byte sent is contained in the receive buffer,
and all other bytes are lost.
Data transfer is initiated by the Master device writ-
ing to its Serial Peripheral Data I/O Register.
Clearing the SPIF bit is accomplished by a soft-
ware sequence which accesses the Serial Periph-
eral Status Register while SPIF is set, followed by
a write or read operation on the Serial Peripheral
Data I/O Register.
In the Master device, while SPIF is set, all writes to
the Serial Peripheral Data I/O Register are inhibit-
ed until the Serial Peripheral Status Register is
read.
In the Slave device, SPIF can be cleared (using a
similar sequence) during a second transmission;
however, it must be cleared before the second
SPIF bit in order to prevent an overrun condition.
The SPIF bit is cleared on Reset.
Bit-6 = WCOL Write Collision status bit
The Write Collision Status bit informs the user that
an attempt was made to write to the Serial Periph-
eral Data I/O Register while a data transfer was
taking place with an external device. The transfer
continues uninterrupted, and therefore a write will
be unsuccessful. A ”read collision” cannot occur,
since the received data byte is placed in a buffer in
which access is always synchronous with the
MCU operation. If a ”write collision” occurs, WCOL
is set but no SPI interrupt is generated. The WCOL
bit is a status flag only.
Clearing the WCOL bit is accomplished by a soft-
ware sequence of accessing the Serial Peripheral
Status Register while WCOL is set, followed by:
1) A read of the Serial Peripheral Data I/O Register
prior to the SPIF bit being set, or
2) A read or write of the Serial Peripheral Data I/O
Register after the SPIF bit is set.
A write to the Serial Peripheral Data I/O Register
(SPDR) prior to the SPIF bit being set, will result in
generation of another WCOL status flag. Both the
SPIF and WCOL bits will be cleared in the same
sequence. If a second transfer has started while
trying to clear the (previously set) SPIF and WCOL
bits with a clearing sequence comprising a write to
the Serial Peripheral Data I/O Register, only the
SPIF bit will be cleared.
A collision of a write to the Serial Peripheral Data
I/O Register while an external data transfer is tak-
ing place can occur both in the Master mode and
the Slave mode, although with proper program-
ming the Master device should have sufficient in-
formation to preclude this collision.
Collision in the Master device is defined as a write
of the Serial Peripheral Data I/O Register while the
internal rate clock (SCK) is in the process of trans-
fer. The signal on the SS pin is always at a logic
high level on the Master device.
Collision in a Slave device is defined in two sepa-
rate modes. A problem arises in a Slave device
when the CPHA control bit is reset. When CPHA is
reset, data is latched on the occurence of the first
clock transition. The Slave device does not have
any way of knowing when that transition will occur;
therefore, the Slave device collision occurs when it
attempts to write the Serial Peripheral Data I/O
Register after its SS pin has been pulled low. If the
CPHA bit is reset, the SS pin on the Slave device
freezes the data in its Serial Peripheral Data I/O
Register and does not allow it to be altered.
The Master device must raise the SS pin of the
Slave device to a logic high level between each
byte it transfers to the Slave device
70
SPIF
WCOL
-
MODF
-
-
-
-


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