Electrical – SPI Half-Duplex for the Holtic HI-3593 ARINC429 IC

duplexspi

I am having some problem understanding half duplex SPI. My understanding of SPI is when you send a byte you read the garbage byte, and if you want to read a value you send a dummy byte to read the value. But for the Holtic HI-3593 chip, this is what it says:

For the HI-3593, each SPI read or write operation begins with an 8-bit
command byte transferred from the host to the device after assertion
of \$\small\overline{CS}\$. Since HI-3593 command byte reception is
half-duplex, the host discards the dummy byte it receives while
serially transmitting the command byte.

My questions are:

Do I need to have an explicit command to read the dummy/garbage byte or is it taken care of?
To read a value, do I need to send a dummy byte?

Best Answer

SPI is logically a circular buffer - I mention this as you seem confused.

On page 14 of the datasheet (behind a script), it states:

In the general case, both master and slave simultaneously send and receive serial data (full duplex), per Figure 5 below. However the HI-3593 operates half duplex, maintaining high impedance on the SO output, except when actually transmitting serial data. When the HI-3593 is sending data on SO during read operations, activity on its SI input is ignored. Figures 6 and 7 show actual behavior for the HI-3593 SO output.

This is not unusual at all (and just about every Holt device I have used that has SPI does this); During command transmit, the SO pin will not be driven at all (the slave SO pin is High-Z); i.e. there is no meaningful data at the host receive pin.

This expands on a comment; to read data, operate the clock normally; and data on SI at the HI-3593 is ignored.

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Electronic – SD card read/write problems in SPI mode

The "checkbusy" must send 0xFF to the SD card (that is not clear in your question). Is this SD or SDHC? The latter have some differences in initialization and operation sequences.

If we don't send dummy data after read ...

In SPI, you have to send data to be able to read from the client. Remember that the master always drives the clock line. The dummy data I send when reading SD cards is 0xFF, too.

The 512 zero bytes may be actual data from the device. Are you sure that you sent the address bytes in the correct order? The SD Simplified Specifications are available from the SD Association.

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.

Best Answer

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Electronic – SD card read/write problems in SPI mode

Spi interrupt code

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