L138 SPI bit length

tehoaislimau

Intellectual 360 points

Hello

I am planning to hookup with a LCD slave device on the SPI for register writes.

The LCD has a 8bit preamble and 16bit data length.

From the Freon specifications it seems that it can only support up to 16bit bit length.

Is there a way for me to send 24bit length to initialize the LCD?

over 15 years ago

0 tlee over 15 years ago

TI__Guru 62975 points

You are correct. SPI supports up to 16-bit characters only. You may be able to use McBSP or McASP instead to transmit 24-bit frames.

0 Gus Martinez over 15 years ago in reply to tlee

TI__Mastermind 29161 points

Tommy,

It's true that the maximum character length of the SPI is 16 bits, but with the use of the CSHOLD feature of the SPI you can achieve any size data transfer including the 24-bit requirement being asked here. The CSHOLD feature is described in the L138 SPI user guide and in this wiki topic: http://processors.wiki.ti.com/index.php/Using_SPI_Chip_Select_Pin_on_C674x/OMAP-L1x.

To get the 24-bit data required by the LCD:

- Set the character length to 8 bits.

- Set CSHOLD = 1 (this forces the CS pin to its active state)

- Transmit 3 8-bit characters.

- Set CSHOLD = 0 (this forces the CS pin to its inactive state.

0 tehoaislimau over 15 years ago in reply to Gus Martinez

Intellectual 360 points

Hi would this method work for a write-read process.

Example,

I would need to write 8-bit address data to the LCD, followed by a 16-bit read data.

0 tlee over 15 years ago in reply to tehoaislimau

TI__Guru 62975 points

The SPI user guide says the following:

Each clock pulse on the SPIx_CLK pin causes the simultaneous transfer (in both directions) of one bit by both the master and slave SPI devices.

So the method should work, but you would need to transmit dummy data to clock in the 16-bit read data. I would have expected the read and write data to be the same data length.

-Tommy

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L138 SPI bit length