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参数资料
| 型号： | ST72621L4T1 |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, MROM, 8 MHz, MICROCONTROLLER, PQFP44 |
| 封装： | TQFP-44 |
| 文件页数： | 92/134页 |
| 文件大小： | 815K |
| 代理商： | ST72621L4T1 |
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SERIAL PERIPHERAL INTERFACE (Cont’d)
10.4.4.6 Write Collision Error
A write collision occurs when the software tries to
write to the SPIDR register while a data transfer is
taking place with an external device. When this
happens, 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
SPIDR register and output the MSBit on to the ex-
ternal 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.
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 SPIDR 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 SPIDR 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 SPICSR 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 5).
Figure 40. Clearing the WCOL bit (Write Collision Flag) Software Sequence
Clearing sequence after SPIF = 1 (end of a data byte transfer)
1st Step
Read SPICSR
Read SPIDR
Write SPIDR
2nd Step
SPIF =0
WCOL=0
SPIF =0
WCOL=0 if no transfer has started
WCOL=1 if a transfer has started
Clearing sequence before SPIF = 1 (during a data byte transfer)
1st Step
2nd Step
WCOL=0
before the 2nd step
Read SPICSR
Read SPIDR
Note: Writing in SPIDR register
instead of reading in it do not re-
set WCOL bit
Read SPICSR
OR
THEN
1
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