Electrical – SPI- Full duplex and Half duplex

spi

Could anyone tell what is the advantages/disadavantages of using half duplex SPI over full duplex SPI communication. I know using half duplex, we can save one pin. But is this the only advantage of using half duplex? Is the rate at which master reads the data from the slave same for both mode of communication?
Please reply

Regards

Best Answer

~~Half duplex~~ unidirectional or write only mode could be used when master don't need data from slave. For example when working with most LCD it is usually OK to do writes only (although by reading status, framebuffer etc master can detect misbehavior and reset the slave, but LCD is usually not critical component and scheduled blind resets might be enough). The advantage is, as you mentioned, saving 1 pin.

Read/write clock rates are always same, since it is done simultaneously. MCUs SPI read and write buffers are shift registers with same clock, so when master is writing it is also reading (even if slave is not sending, in that case HIGH bits are received).

Related Solutions

Spi interrupt code

With SPI the master generates the clock so there's not really a concept of being able to receive multiple bytes back from a slave automatically, you have to send additional dummy bytes to generate the clock. Say if you had a sensor and you wanted command 0x01 to send back the current temperature the pseudo-code for the master may look like:

spi_write(0x01);
t_lsb = spi_write(0x00); // These dummy bytes are just needed to generate a clock
t_msb = spi_write(0x00); // So that the slave gets a chance to send the results back

On and the slave it might look like:

cmd = spi_read();
if (cmd == 0x01) {
  spi_write(t_lsb);
  spi_write(t_msb);
}

You can technically send back a byte from the slave for each byte sent from the master, but for most protocols there probably isn't anything useful to send back until the slave has received and decoded the command. Hopefully you can understand that pseudo-code and the general concept OK, it's been a while since I've programmed PIC SPI at the register level but it sounds like you're on top of that side of things.

SPI Slave Frequency Setup

No, it doesn't. SPI is a synchronous protocol. That means that you transmit the clock as a discrete signal and it is used directly by the receiver to time the reception of the data. The receiver will have a maximum frequency at which it can operate, and some (such as some ADCs) will have a minimum frequency to maintain internal accuracy.

Best Answer

Related Solutions

Spi interrupt code

SPI Slave Frequency Setup

Related Topic