Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
162
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare
the data to be sent in their respective shift Registers, and the Master generates the required clock pulses on the
SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI,
line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will
synchronize the Slave by pulling high the Slave Select, SS, line.
the data to be sent in their respective shift Registers, and the Master generates the required clock pulses on the
SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI,
line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will
synchronize the Slave by pulling high the Slave Select, SS, line.
When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled
by user software before communication can start. When this is done, writing a byte to the SPI Data Register
starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the
SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in
the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it
into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be
kept in the Buffer Register for later use.
by user software before communication can start. When this is done, writing a byte to the SPI Data Register
starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the
SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in
the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it
into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be
kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS pin is
driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not
be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been
completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR
Register is set, an interrupt is requested. The Slave may continue to place new data to be sent into SPDR
before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use.
driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not
be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been
completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR
Register is set, an interrupt is requested. The Slave may continue to place new data to be sent into SPDR
before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use.
Figure 19-2.
SPI Master-slave Interconnection
The system is single buffered in the transmit direction and double buffered in the receive direction. This means
that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed.
When receiving data, however, a received character must be read from the SPI Data Register before the next
character has been completely shifted in. Otherwise, the first byte is lost.
that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed.
When receiving data, however, a received character must be read from the SPI Data Register before the next
character has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct sampling
of the clock signal, the minimum low and high periods should be:
of the clock signal, the minimum low and high periods should be:
Low periods: Longer than 2 CPU clock cycles.
High periods: Longer than 2 CPU clock cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to
. For more details on automatic port overrides, refer to
Table 19-1.
SPI Pin Overrides
Pin
Direction, Master SPI
Direction, Slave SPI
MOSI
User Defined
Input
SHIFT
ENABLE
ENABLE