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ST7285C Datasheet(HTML) 52 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) 52 Page - STMicroelectronics

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ST7285C
SERIAL PERIPHERAL INTERFACE(Cont’d)
The second collision mode is defined by the CPHA
control bit being set. With the CPHA bit set, the
Slave device will be receiving a clock (SCK) edge
prior to the latch of the first data transfer. This first
clock edge will freeze the data in the Slave device
I/O register and allow the msb on to the external
MISO pin of the Slave device. A logic low state on
the SS pin enables the Slave device, however,
data is not output on the MISO pin until the first
edge of the data transfer clock. The WCOL bit will
only be set if the I/O register is accessed while a
transfer is taking place. In this second collision
mode, a Master device can hold a Slave device’s
SS pin low during the transfer of several bytes of
data without this causing any problems.
Unlike other SPI interfaces, there is no special
case of collision which remains undetected by
the WCOL bits. The WCOL bit is TOTALLY reli-
able with regard to collision detection.
Since the Slave device is operating asynchronous-
ly with the Master device, the WCOL bit may be
used as an indicator of a collision occurence. This
helps alleviate the user from a strict real-time pro-
gramming effort. The WCOL is cleared on Reset.
Bit-4 = MODF
Mode Fault flag
The function of the mode fault flag is defined for
the Master mode (device). If the device is a Slave
device the MODF bit will be prevented from tog-
gling from reset to set; however, this does not pre-
vent the device from being in the Slave mode with
the MODF bit set.
The MODF bit is normally reset, and is set only
when the Master device has itsSS pin pulled low.
Toggling the MODF bit to the set state affects the
internal Serial Peripheral Interface (SPI) system in
the following ways:
a) MODF is set and SPI interrupt is generated if
SPIE is set.
b) The SPE bit is forced to a reset state. This
blocks all output from the device and disables the
SPI system.
c) The MSTR bit is forced to a reset state, thus
forcing the device into the Slave mode.
Clearing the MODF is accomplished by a software
sequence which accesses the Serial Peripheral
Status Register while MODF is set, followed by a
write to the Serial Peripheral Control Register.
To avoid multiple Slave conflicts in a system com-
prising several MCUs, the SS pin must be pulled
high during the clearing sequence of MODF. Con-
trol bits SPE and MSTR may be restored to their
original set state during this clearing sequence, or
after the MODF bit has been cleared. Hardware
does not allow the user to set the SPE and MSTR
bits while MODF is set, unless this occurs during
the proper clearing sequence. The MODF flag bit
indicates the possible occurrence of a Multimaster
conflict for system control, and allows proper exit
from normal system operation to Reset or to a de-
fault system state. The MODF bit is cleared on Re-
set.


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