ST72T311 ST Microelectronics, ST72T311 Datasheet - Page 70

• Manufacturer Part Number: ST72T311
• Description: 8-BIT MCU WITH 8 TO 16K OTP/EPROM
• Manufacturer: ST Microelectronics

ST72E311 ST72T311
SERIAL PERIPHERAL INTERFACE (Cont’d)
4.5.4.4 Write Collision Error
A write collision occurs when the software tries to
write to the DR register while a data transfer is tak-
ing place with an external device. When this hap-
pens, the transfer continues uninterrupted; and
the software write will be unsuccessful.
Write collisions can occur both in master and slave
mode.
Note: a ”read collision” will never occur since the
received data byte is placed in a buffer in which
access is always synchronous with the MCU oper-
ation.
In Slave mode
When the CPHA bit is set:
The slave device will receive 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
DR register and output the MSBit on to the exter-
nal MISO pin of the slave device.
The SS pin low state enables the slave device but
the output of the MSBit onto the MISO pin does
not take place until the first data transfer clock
edge.
Figure 41. Clearing the WCOL bit (Write Collision Flag) Software Sequence
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1st Step
2nd Step
Clearing sequence after SPIF = 1 (end of a data byte transfer)
Clearing sequence before SPIF = 1 (during a data byte transfer)
70
1st Step
2nd Step
Read DR
Read SR
THEN
SPIF =0
WCOL=0
Read DR
Read SR
OR
THEN
WCOL=0
When the CPHA bit is reset:
Data is latched on the occurrence 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
software attempts to write the DR register after its
SS pin has been pulled low.
For this reason, the SS pin must be high, between
each data byte transfer, to allow the CPU to write
in the DR register without generating a write colli-
sion.
In Master mode
Collision in the master device is defined as a write
of the DR register while the internal serial clock
(SCK) is in the process of transfer.
The SS pin signal must be always high on the
master device.
WCOL bit
The WCOL bit in the SR register is set if a write
collision occurs.
No SPI interrupt is generated when the WCOL bit
is set (the WCOL bit is a status flag only).
Clearing the WCOL bit is done through a software
sequence (see Figure 41).
Read SR
Write DR
Note: Writing in DR register in-
stead of reading in it do not reset
WCOL bit
THEN
SPIF =0
WCOL=0
WCOL=1
before the 2nd step
if no transfer has started
if a transfer has started