Serial communication between PIC and components

[vinniewryan]

For the last week I've been looking for a tutorial on working with serial data between microcontrollers and serial components. If anyone knows of a good tutorial that spells out the process of working with serial communications with microcontrollers, please enlighten me. I would love to learn everything about serial data exchange, but for now I need to learn what I can to get past my problem.

Basically I have an ADXL345Z digital accelerometer that communicates with a microcontroller via serial data. The data sheet for the accelerometer can be found here: http://www.analog.com/static/imported-files/data_sheets/ADXL345.pdf

The PINOUT for the accelerometer as mounted on the EVAL board is as follows:

1 VIO - I/O Supply Voltage
2 GND - Ground
3 CS - Chip Select
4 VS - Supply Voltage (3.0V)

5 SCL - Serial Clock
6 SDA/SDI - Serial Data Input
7 SDO - Serial Data Output
8 INT1 - Interrupt 1
9 INT2 - Interrupt 2

Instead of analog X Y Z pins, I have to retrieve the X Y Z values through SDI and SDO serial I/O pins. This is all fine except I'm not too entirely familiar with how to do this. I understand there is a process for working with serial data that goes something like this:

• Initiate clock between PIC and device
• Send X Y Z values to registers
• Read data from registers to the PIC
• Convert data to numeric values
• Repeat multiple times per second

This is where I'm stumped. I've tried to find help online, and I've had some success with learning about registers but I don't know how to do what I need to do with the knowledge I have. If anyone here has the knowledge to help me get started, I would be very grateful. Starting with how to set a pin on my PIC as a serial data input pin, and another as a serial output pin. I'm using a 16F684 14 pin 8 bit PIC.

Thanks!

[vinniewryan]

I think I'm on the right track:

[andrewg]

Your accelerometer is not an RS232 serial device, but an SPI serial device. Look up "#use spi" in your manual...
_________________
Andrew

[ckielstra]

As an additional source of info study the SPI article on Wikipedia.

A confusing part in the SPI communication is that there are 4 different possible configurations but Motorola (Freescale) never defined this very well resulting in each hardware vendor using their own names (license issues also have to do with this).

This is the translation table I always use:

[vinniewryan]

Wow that cleared so many things up! Thanks for the reply. I just finished going over the SPI functions in the manual, as well as the examples. I think I have it figured out except for setting the input pin to A0. Which is correct?

[ckielstra]

[mkuang]

In your drivers directory see whether you see a file called 9356.c. It shows you how to write to the 9356 series eeprom which use spi, on a PIC that does not have hardware spi.

Basically you have to clock the data out yourself, pretty tedious but not difficult. Personally I wouldn't pick an device nowadays without hardware spi (or uart for that matter).

[vinniewryan]

This is actually a lot more simple than I thought. I've been reading and messing around with the test codes some more and I came up with what I think will be a working program, though when I compile it, I get these errors:

"Undefined identifier -- setup_spi"
"Undefined identifier -- spi_read"

[andrewg]

setup_spi, spi_data_is_in and spi_read are all only used with hardware SPI, which your PIC doesn't have.

In your case #use spi will setup software SPI support, and then you just use the function spi_xfer to transfer data.

Typical interaction with an SPI device involves setting the chip select to select the device, performing the comms, then deselecting the device. This last can be important as some devices won't act on commands sent to them until they've been deselected.

You may not even need the output_low calls to setup the input, output and clock pins. I think the compiler generates code to do that. You'll need to keep the LED and chip select setups for sure, though. If you inspect the compiler listing, you could probably confirm the code generated for the #use spi is setting up the outputs.
_________________
Andrew

[vinniewryan]

I'm close, I think. Thanks again for helping me through this. I don't know what I would have done without it your aid.

The only thing I'm having trouble with now is reading/ writing to and from the registers. I've written the following sample program that doesn't work, and I'm sure it's obvious why, but I can't figure out the proper way to do read or write from the accelerometer via SPI. The only thing required by the accelerometer is that I bring it out of standby mode by setting the FIFO bit. I'm not sure what to set it to but the data sheet also said that 3 is xyz measure mode, which is what I want. So I tried this:

spi_xfer(3), (0x38); //FIFO CTRL bit, bit 3 = xyz measure mode bit

And I failed. Details below, this is the program:

[andrewg]

[vinniewryan]

[andrewg]

accel_reg_read is reading the value of a register in the accelerometer and returning that value so your app can doing something with it.

The "|0x80" part is setting bit 7 in the first byte sent to the accelerometer. If you read the data sheet (page 9), you'll see there's a bit marked R/W with a line over the W. What that means is when that bit is 1, the command is a Read and when it's 0, it's a Write.

The next bit is "MB" which is "Multi-Byte". I've ignored that and left it zero (single-byte transfers). The rest of the bits are address and data arguments.

To switch the accelerometer into measure mode, the code would be:

[vinniewryan]

I tried compiling the code you provided, and when the compiler asked for a main function I re-formatted the code to implement one. Now the LED isn't even coming on. I've tried messing around with it a little bit, but I can't seem to get the accelerometer to read anything. I'm not sure what the problem is...

[andrewg]

Take accel_reg_read out of the main function. C doesn't allow nested functions.
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Andrew