MEC1322 Datasheet, PDF (317/456 Pages) Microchip Technology – Keyboard and Embedded Controller for Notebook PC

MEC1322
• A dummy 8 bit data value (any value) 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 transmit bytes:
- 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 invalid since the last cycle was initiated solely to
transmit command data to the slave. This particular slave device drives '0' on the SPDIN pin to the master
while it is accepting command data. This SPIRD data is ignored.
- Once the first SPI cycle is completed, the SPI master takes the pending data in the TX_DATA register 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 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.
• The final SPI cycle is initiated when another dummy 8 bit data value (any value) is written to the TX_DATA regis-
ter. Note that this value may be another dummy value or it can be a new 8 bit command to be sent to the ADC.
The new command will be transmitted while the final data from the last command is received simultaneously. This
overlap allows ADC data to be read every 16 SPCLK cycles after the initial 24 clock cycle.The SPI master auto-
matically 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, the second SPI cycle is complete:
- 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 a valid 16 bit ADC value. SPIRD is
read and stored.
- Once the second SPI cycle is completed, the SPI master takes the pending data in the TX_DATA register and
loads it into the TX shift register. Loading the shift register automatically asserts the TXFE bit, begins shifting
the data value onto the SPDOUT pin, and drives the SPCLK pin. Data on the SPDIN pin is also sampled on
each clock.
• After 8 SPI_CLK pulses, the final SPI cycle is complete, TXBF is asserted '1', and the SPINT interrupt is asserted
(if enabled). The data now contained in SPIRD - SPI RX_Data Register is the second half of a valid 16 bit ADC
value. SPIRD is read and stored.
• If a command was overlapped with the received data in the final cycle, #CS should remain asserted and the SPI
master will initiate another SPI cycle. If no new command was sent, #CS is released and the SPI is idle.
27.11.1.2 Read/Write
The slave device used in this example is a Fairchild NS25C640 FM25C640 64K Bit Serial EEPROM. The following sub-
sections describe the read and write sequences.
Read
• The SPI block is activated by setting the enable bit in SPIAR - SPI Enable Register
• The SPIMODE bit is de-asserted '0' to enable the SPI interface in Full 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.
• Assert CS# low using a GPIO pin.
• Write a valid command word (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. 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.
• Next, EEPROM address A15-A8 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.
 2014 - 2015 Microchip Technology Inc.
DS00001719D-page 317