Electronic – Correct SPI Configuration

avr32schematicsspi

I'm still not entirely clear on TTL, so please, forgive my ignorance in this.

I have a sensor and a microcontroller. The sensor is a BMI055 Gyroscope and accelerometer. The microcontroller is the Atmel AT32UC3C0512CAU-ALUT. With these two, I want to connect them with a correct 4-wire SPI interface.

Here is my proposed layout:

pins 13-17 are as follows from the datasheet (above):

13: (Function B) SPI0 MOSI

14: (Function B) SPI0 MISO

15: (Function B) SPI0 SCK

16: (Function B) SPI0 NPCS{0} (NPCS is chip select pin)

17: (Function B) SPI0 NPCS{1}

Thing I have questions on: Am I correct in using pull-down resistors? I hear Pull-up is more common, but I'm not sure why. Another thing is how would you make this schematic prettier? it's pretty bad right now. and my final concern is the value of these resistors. I'm not sure how to calculate that.

Best Answer

SPI uses a push-pull configuration, so you don't really need the resistors.

The resistors are only there to have a defined level while the pins of the MCU are configured as an input. So in this case you can use pretty large resistors (like 100k) to reduce the additional current draw of the resistors when you are actively driving the lines.

The direction in which you pull the line depends on the idle state of the connected line. If it is idle high, you use a pull-up, if it is idle low, you use a pull-down.

So for the BMI055 it seems like pulling CSx high would be better and SCK high as well (both idle high).

You can probably remove the diodes as the SDO state is High-Z (high impedance) when not active, so you won't get a short.

You are not supplying a voltage to VDDIO.

Place 100nF caps in parallel to the ones you have on VDD and VDDIO.

Ways to improve your schematic (in my view, as with coding styles, they differ from person to person):

make sure there is a junction point on every point a connection is made (there are missing a some on the pull-down resistors for example)
place the reference symbols in the "right" direction, GND points downwards, 3V3 upwards
Don't place parts of your question on your schematic only
Where are all the arrows going?
Laying out the components pins the way they are in the datasheet helps a peer greatly to do a review, currently it's hard to tell if they are right (also the names should be the same or more explanatory than those in the datasheet)
I miss the actual pin numbers (cannot check if labels and numbers are correctly matched to each other)

Related Solutions

Electronic – Arduino SPI Clock Speed Configuration

You can set the SPI speed (and many other things!) using the Arduino SPI library without specifying any register values. You already did it with MSBFIRST. Like so:

  SPI.setDataMode(SPI_MODE3);
  SPI.setBitOrder(MSBFIRST);
  SPI.setClockDivider(SPI_CLOCK_DIV4); // 16/4 MHz
  SPI.begin();

Also, SPI.begin sets all the pin modes for you, so no need to do that.

Electronic – How to receive multiple bytes on SPI with CS being held low

I did the same thing on a AT32UC3C0512 (yet, I was the master which probably made it a bit easier).

The idea is probably the following (if you're using the ASF framework, it should work in slave mode as well):

Call spi_get(SPI)
Loop until spi_is_rx_ready(SPI) or until you run into a timeout
Call spi_get(SPI) for the next byte (or 10 byte or whatever you set the spi_rx register size to be)
Do this a few times until you received all bytes

Here's some code as a starting point, hope it helps a little:

volatile avr32_spi_t *spi = device->spi;
unsigned int timeout = SPI_TIMEOUT;    

timeout = SPI_TIMEOUT;
while (!spi_is_rx_ready(spi)) {
    if (!timeout--) {
        tmc260_deselect_device(device);
        return ERR_TIMEOUT;
    }
}
// Store the received data
rx_msb = spi_get(spi);

I'm not quite sure what has to be done to reset the flag which is set once a byte was received. I would assume simply reading the rx register should do the trick.

It should also be possible to port this code to using interrupts (call the ISR whenever a byte was received/RX register is full).

From the AT32UC3C0512 datasheet:

The SPI waits for NSS to go active before receiving the serial clock from an external master. When NSS falls, the clock is validated on the serializer, which processes the number of bits defined by the Bits Per Transfer field of the Chip Select Register 0 (CSR0.BITS). These bits are processed following a phase and a polarity defined respectively by the CSR0.NCPHA and CSR0.CPOL bits. Note that the BITS, CPOL, and NCPHA bits of the other Chip Select Registers have no effect when the SPI is configured in Slave Mode. The bits are shifted out on the MISO line and sampled on the MOSI line. When all the bits are processed, the received data is transferred in the Receive Data Register and the SR.RDRF bit rises. If the RDR register has not been read before new data is received, the SR.OVRES bit is set. Data is loaded in RDR even if this flag is set. The user has to read the SR register to clear the SR.OVRES bit.

As far as I understand, a flag will be set after 8 to 10 databits. For a first try, you will need to read the data and see if the flag gets raised a second time, even if CS doesn't get asserted a second time.

An alternative solution

You can also use an interrupt on the SCK line (depending on the mode on the rising or falling edge) and sample the data "manually". Once you discover a condition where SCK rises (or falls) and CS is high, you are outside the "normal" datatransfer and you start a new reception.

Best Answer

Related Solutions

Electronic – Arduino SPI Clock Speed Configuration

Electronic – How to receive multiple bytes on SPI with CS being held low

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