Things to set up when using the HC12 SPI subsystem

• Enable SPI
• Master or Slave?
  • Master generates clock for data transfers; slave uses master's clock
• MSB first or LSB first?
  • Normally, MSB first
• Clock Polarity
  • Clock idle low or clock idle high?
• Clock Phase
  • Data valid on first clock edge or second clock edge?
• Clock Speed (set by Master)
• Generate interrupt after data transfered?
• Bidirectional Mode (we will not use)
• Wired-OR Mode (we will not use)

Use the following registers:

SP0CR1, SP0CR2, SP0BR, SP0SR, SP0DR, DDRS

1.

Enable SPI (SPE bit of SP0CR1)

2.

Clock phase and polarity set to match device communicating with

3.

Select clock polarity - CPOL bit of SP0CR1

• CPOL = 0 for clock idle low
• CPOL = 1 for clock idle high

4.

Select clock phase - CPHA bit of SP0CR1

• CPHA = 0 for data valid on first clock edge
• CPHA = 1 for data valid on second clock edge

5.

Select master or slave MSTR bit of SP0CR1

• Will be master when talking to devices such as D/A, A/D, clock, etc.
• May be slave if talking to another microprocessor

6.

If you want to receive interrupt after one byte transfered, enable interrupts with SPIE bit of SP0CR1

• Normally master will not use interrupts - transfers are fast enough that you will normally wait for transfer to complete
• Will often use interrupts when configured as a slave - you will get interrupt when master sends you data

7.

Configure LSBF of SP0CR1 for MSB first (LSBF = 0) or LSB first (LSBF = 1)

• For most devices, use MSB first

8.

Configure for normal mode by clearing bit SPC0 of SP0CR2

• Bidirectional mode (SPC1 = 1 in SP0CR2) used for three-wire communication - need some protocol for selecting who is sender and who is receiver

Master Mode:

1.

Set clock rate - SPR2:0 bits of SP0BR

• Normally select clock at highest rate compatible with slave

2.

Make MOSI, SCLK, and SS output - bits DDS5, DDS6, DDS7 of DDRS

3.

MISO automatically configured as input by choosing master mode

4.

Configure some way to select slave(s) - probably SS if only one slave; other I/O bits if multiple slaves

5.

Start data transfer by writing byte to SP0DR

6.

After transfer complete (8 clock cycles), SPIF bit of SP0SR set.

• If writing data to slave, can send next byte to SP0DR
• If reading data from slave, can read data from SP0DR

7.

Set up SSOE of SP0CR1

• SSOE = 0 if you want to control SS yourself (to be able to send more than one byte with SS low)
• SSOE = 1 if you want to SS controlled automatically (SS will be active for one byte at a time)

Slave Mode:

1.

No need to set clock mode - slave accepts data at rate sent by master (up to 4 MHz)

2.

Need to make MISO output - bit DDS4 of DDRS

3.

No need to Make MOSI, SCLK, and SS inputs - this is done automatically when configuring HC12 as slave

• If receiving data from master, wait until SPIF flag of SP0SR set (or until SPI interrupt received), then read data from SP0DR
• If sending data to master, write data to SP0DR before master starts transfer