MEC1322 Datasheet, PDF (320/456 Pages) Microchip Technology – Keyboard and Embedded Controller for Notebook PC

MEC1322
SPI RX_Data Register is invalid since the last cycle was initiated solely to transmit address data to the slave.
- Once the third SPI cycle is completed, the SPI master takes the pending data in the TX_DATA register (data
byte D7:D0) and loads it into the TX shift register. Loading the shift register automatically asserts the TXFE
bit, begins shifting the dummy data value onto the SPDOUT pin, and drives the SPCLK pin. Data on the
SPDIN pin is also sampled on each clock.
• Once the TXFE bit is asserted the SPI Master is ready to receive its next byte. Before writing the next TX_DATA
value, software must clear the RXBF status bit by reading the SPIRD - SPI RX_Data Register.
• If only one data byte is to be written, the host would not write any more values to the TX_DATA register until this
transaction completes. If more than one byte of data is to be written, another data byte would be written to the
TX_DATA register. The SPI master automatically clears the TXFE bit when the TX_DATA register is written, but
does not begin shifting this data value onto the SPDOUT pin. This byte will remain in the TX_DATA register until
the TX shift register is empty.
• After 8 SPI_CLK pulses, the fourth SPI cycle is complete (First Data Byte transmitted):
- The data byte has been transmitted to the slave completing the fourth SPI cycle. Once again, the RXBF bit is
asserted '1' and the SPINT interrupt is asserted, if enabled. Like the command and address phases, the data
now contained in SPIRD - SPI RX_Data Register is invalid since the last cycle was initiated to transmit data to
the slave.
- Once the fourth SPI cycle is completed, the SPI master takes the pending data in the TX_DATA register (if
any) and loads it into the TX shift register. This process will be repeated until all the desired data is transmit-
ted.
• If no more data needs to be transmitted by the master, CS# and WR# are released and the SPI is idle.
27.11.2 HALF DUPLEX (BIDIRECTIONAL MODE) TRANSFER EXAMPLE
The slave device used in this example is a National LM74 12 bit (plus sign) temperature sensor.
• The SPI block is activated by setting the enable bit in SPIAR - SPI Enable Register
• The SPIMODE bit is asserted '1' to enable the SPI interface in Half Duplex mode.
• The CLKPOL, TCLKPH and RCLKPH bits are de-asserted '0' to match the clocking requirements of the slave
device.
• The LSBF bit is de-asserted '0' to indicate that the slave expects data in MSB-first order.
• BIOEN is asserted '0' to indicate that the first data in the transaction is to be received from the slave.
• Assert CS# using a GPIO pin.
//Receive 16-bit Temperature Reading
• Write a dummy command byte (as specified by the slave device) to the SPITD - SPI TX_Data Register with TXFE
asserted '1'. The SPI master automatically clears the TXFE bit indicating the byte has been put in the TX buffer. If
the shift register is empty the TX_DATA byte is loaded into the shift register and the SPI master reasserts the
TXFE bit. Once the data is in the shift register the SPI master begins shifting the data value onto the SPDOUT pin
and drives the SPI_CLK pin. This data is lost because the output buffer is disabled. Data on the SPDIN pin is sam-
pled on each clock.
• Once the TXFE bit is asserted the SPI Master is ready to receive its next byte. Before writing the next TX_DATA
value, software must clear the RXBF status bit by reading the SPIRD - SPI RX_Data Register.
• Next, another dummy byte is written to the TX_DATA register. The SPI master automatically clears the TXFE bit,
but does not begin shifting the dummy data value onto the SPDOUT pin. This byte will remain in the TX_DATA
register until the TX shift register is empty.
• After 8 SPI_CLK pulses from the first receive byte
- The first SPI cycle is complete, RXBF bit is asserted '1', and the SPINT interrupt is asserted, if enabled. The
data now contained in SPIRD - SPI RX_Data Register is the first half of the 16 bit word containing the tem-
perature data.
- Once the first SPI cycle is completed, the SPI master takes the pending data in the TX_DATA register
(dummy byte 2) and loads it into the TX shift register. Loading the shift register automatically asserts the
TXFE bit, begins shifting the dummy data value onto the SPDOUT pin, and drives the SPI_CLK pin. Data on
the SPDIN pin is also sampled on each clock.
• Once the TXFE bit is asserted the SPI Master is ready to receive its next byte. Before writing the next TX_DATA
value, software must clear the RXBF status bit by reading the SPIRD - SPI RX_Data Register.
//Transmit next reading command
DS00001719D-page 320
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