UART
The UART in the AT89S8252 operates the same way as the UART in the AT89C51 and
AT89C52. For further information on the UART operation, refer to the Atmel web site
(http://www.atmel.com). From the home page, select “Products”, then “Microcontrollers,
then “8051-Architecture”. Click on “Documentation”, then on “Other Documents”. Open
the document “AT89 Series Hardware Description”.
Serial Peripheral
Interface
The serial peripheral interface (SPI) allows high-speed synchronous data transfer
between the AT89S8252 and peripheral devices or between several AT89S8252
devices. The AT89S8252 SPI features include the following:
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Full-Duplex, 3-Wire Synchronous Data Transfer
Master or Slave Operation
1.5 MHz Bit Frequency (max.)
LSB First or MSB First Data Transfer
Four Programmable Bit Rates
End of Transmission Interrupt Flag
Write Collision Flag Protection
Wakeup from Idle Mode (Slave Mode Only)
The interconnection between master and slave CPUs with SPI is shown in the following
figure. The SCK pin is the clock output in the master mode but is the clock input in the
slave mode. Writing to the SPI data register of the master CPU starts the SPI clock gen-
erator, and the data written shifts out of the MOSI pin and into the MOSI pin of the slave
CPU. After shifting one byte, the SPI clock generator stops, setting the end of transmis-
sion flag (SPIF). If both the SPI interrupt enable bit (SPIE) and the serial port interrupt
enable bit (ES) are set, an interrupt is requested.
The Slave Select input, SS/P1.4, is set low to select an individual SPI device as a slave.
When SS/P1.4 is set high, the SPI port is deactivated and the MOSI/P1.5 pin can be
used as an input.
There are four combinations of SCK phase and polarity with respect to serial data,
which are determined by control bits CPHA and CPOL. The SPI data transfer formats
are shown in Figure 8 and Figure 9.
Figure 7. SPI Master-slave Interconnection
MSB MASTER
8-BIT SHIFT REGISTER
LSB
MSB
SLAVE
LSB
MISO MISO
MOSI MOSI
8-BIT SHIFT REGISTER
SCK
SS
SCK
SS
SPI
CLOCK GENERATOR
VCC
18
AT89S8252
0401G–MICRO–3/06
ATMEL [ ATMEL ]
