SPI communication between PICs - Revision history

Gregory McGlynn at 07:53, 5 May 2009

2009-05-05T07:53:29Z

Alex Park: /* Code */

2009-02-12T06:38:33Z

Code

Alex Park: /* Overview */

2009-02-12T06:37:42Z

Overview

Alex Park: /* Code */

2009-02-12T06:36:37Z

Code

Kwang Sim at 09:40, 11 February 2009

2009-02-11T09:40:11Z

Kwang Sim at 09:34, 11 February 2009

2009-02-11T09:34:45Z

Kwang Sim at 09:33, 11 February 2009

2009-02-11T09:33:52Z

Kwang Sim at 09:33, 11 February 2009

2009-02-11T09:33:06Z

Kwang Sim at 09:23, 11 February 2009

2009-02-11T09:23:49Z

Kwang Sim at 09:22, 11 February 2009

2009-02-11T09:22:13Z

← Older revision		Revision as of 07:53, 5 May 2009
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	<b>Two-way communication</b>		<b>Two-way communication [This code has not been successfully tested!]</b>

	Though not shown above, it is possible to use SPI for two-way communication between master and slave. If the slave calls		Though not shown above, it is possible to use SPI for two-way communication between master and slave. If the slave calls

← Older revision		Revision as of 06:38, 12 February 2009
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	[http://www.ccsinfo.com/forum/ CCS forum]		[http://www.ccsinfo.com/forum/ CCS forum]

			[http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus Wikipedia article]

← Older revision		Revision as of 06:37, 12 February 2009
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	[http://www.mct.net/faq/spi.html More Information] (www.mct.net)		[http://www.mct.net/faq/spi.html More Information] (www.mct.net)


	== Circuit ==		== Circuit ==

@@ Line 72: / Line 72: @@
 SPI communication on the PIC is accomplished via the functions spi_read() and spi_write(), which read and write one byte at a time over SPI. In the example below, the master reads an 8-bit value from the analog-to-digital converter and sends it to the slave via SPI. The slave reads the value and displays it on an LCD display.
-<b>Note:</b> Our code did not work perfectly: the slave failed to received 10-20% of the messages sent by the master. In this instance that failure rate is not critical, as the slave just misses an update of the ADC reading when it misses a message. If you need accurate data transmission this is an unacceptable failure rate, however. We are not sure why this occurred.
+<b>Note:</b> The code given below does not work perfectly: the slave failed to received 10-20% of the messages sent by the master. In this example that failure rate was not critical, as the slave just missed an update of the ADC reading when it missed a message. If more accurate data transmission is required, the failure rate inherent in this system may be unacceptable.
-<b>SPI Modes</b>
+<b>SPI Mode Numbers</b>
-There are four SPI "modes" according to the relationship between the phase of the clock line and the phase of the MISO/MOSI lines. [http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus Wikipedia] has a good explanation of what the modes are. The important point is that if you are communicating with a device that uses mode 3, for example, you need to use mode 3 as well to be compatible with it.
+There are four SPI "modes" which describe the relationship between the phase of the clock line and the phase of the MISO/MOSI lines. In order to successfully communicate using SPI, the master and slave must operate using the same mode. [http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus#Mode_Numbers Wikipedia] has a good explanation of these mode numbers.
 <b>The Slave Select Line</b>
-The slave select (SS) line serves two purposes: not only is it used to select which slave is active during a communication, it is also used for synchronization. A slave's SS line should be high except when the master is talking to it. When the SS line is pulled low, the slave knows that a new communication is starting. If a slave somehow fails to receive a message properly, it will be reset when the SS line goes high at the end of the message, and be fresh and ready for a new message when the SS line goes low again. If you don't use an SS line (e.g. by disabling it or running it directly to ground) you run the risk of the slave getting out of sync with the master: if the slave misses a bit, it will always be one bit off in the future. Using the SS line allows the slave and master to realign themselves at the beginning of each communication, in case they get out of sync.
+In addition to being used to select which slave is active during a communication, the slave select (SS) line, is vital for synchronization. A slave's SS line should be high except when the master is talking to it. When the SS line is pulled low, the slave knows that a new communication is starting.
+If a slave fails to receive a message properly, it will be reset when the SS line goes high at the end of the message and the slave will be prepared for a new message when the SS line goes low again. If the SS line is not used (e.g. by disabling it or running it directly to ground) there is a risk of the slave becoming out of sync with the master; if the slave misses a bit, it will always be one bit off in the future. Effective use of the SS line allows the slave and master to realign themselves at the beginning of each communication, in case of a transmission error.
@@ Line 91: / Line 93: @@
 #use delay(clock=40000000)
-//these define the different SPI modes in terms of constants the compiler knows about
+//These define the different SPI modes in terms of constants the compiler knows about
 //NOTE: our PICs only seemed to work in modes 1 and 3, though they are supposed to work with any modes
 #define SPI_MODE_0 (SPI_L_TO_H | SPI_XMIT_L_TO_H)
@@ Line 98: / Line 100: @@
 #define SPI_MODE_3 (SPI_H_TO_L | SPI_XMIT_L_TO_H)
-#define SS_PIN PIN_D0 //this can be any output pin on the master
+#define SS_PIN PIN_D0         //this can be any output pin on the master
 void main()
@@ Line 104: / Line 106: @@
    int val;
-   //set up the ADC to read from A0
+   //Set up the ADC to read from A0
    setup_adc_ports(AN0);
    setup_adc(ADC_CLOCK_INTERNAL);
    set_adc_channel(0);
-   //the next statement sets up the SPI hardware, with this PIC as a master using mode 1
+   //The next statement sets up the SPI hardware, with this PIC as a master using mode 1
    //SPI_CLK_DIV_64 sets the speed of the SPI clock pulses--this is the slowest speed
    setup_spi(SPI_MASTER | SPI_MODE_1 | SPI_CLK_DIV_64);

@@ Line 47: / Line 47: @@
 <b>References</b>
-[http://www.totalphase.com/support/articles/article03/#modes SPI Background].www.totalphase.com
+[http://www.totalphase.com/support/articles/article03/ SPI Background](www.totalphase.com)
-[http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus Serial Peripheral Interface]. Www.wikipedia.org
+[http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus SPI Wikipedia Article] (www.wikipedia.org)
+[http://www.mct.net/faq/spi.html More Information] (www.mct.net)
-[http://www.mct.net/faq/spi.html mct.net]
 == Circuit ==
+<b>Master connected to three slaves:</b>
-see [http://www.totalphase.com/support/articles/article03/#modes www.totalphase.com]
 [[Image:spi-diagram.png]]
-Above shows the Master connected to three slaves. Each slave must be enabled through the slave select pin in order to communicate with the Master.
+Here each slave must be enabled through the slave select pin in order to communicate with the Master.
+<b>One PIC master and one PIC slave:</b>
 [[Image:spi.jpg]]
+The master, on the left, and the slave, on the right, are connected as shown above. Be sure you also connect grounds!
-The two PICs can be wired directly by connecting these input and output pins of the diagram above.
 == Code ==

@@ Line 69: / Line 69: @@
 SPI communication on the PIC is accomplished via the functions spi_read() and spi_write(), which read and write one byte at a time over SPI. In the example below, the master reads an 8-bit value from the analog-to-digital converter and sends it to the slave via SPI. The slave reads the value and displays it on an LCD display.
 <b>SPI Modes</b>
 There are four SPI "modes" according to the relationship between the phase of the clock line and the phase of the MISO/MOSI lines. [http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus Wikipedia] has a good explanation of what the modes are. The important point is that if you are communicating with a device that uses mode 3, for example, you need to use mode 3 as well to be compatible with it.
 <b>The Slave Select Line</b>
 The slave select (SS) line serves two purposes: not only is it used to select which slave is active during a communication, it is also used for synchronization. A slave's SS line should be high except when the master is talking to it. When the SS line is pulled low, the slave knows that a new communication is starting. If a slave somehow fails to receive a message properly, it will be reset when the SS line goes high at the end of the message, and be fresh and ready for a new message when the SS line goes low again. If you don't use an SS line (e.g. by disabling it or running it directly to ground) you run the risk of the slave getting out of sync with the master: if the slave misses a bit, it will always be one bit off in the future. Using the SS line allows the slave and master to realign themselves at the beginning of each communication, in case they get out of sync.
 <b>Master Code:</b>
@@ Line 188: / Line 193: @@
 <b>Two-way communication</b>
 Though not shown above, it is possible to use SPI for two-way communication between master and slave. If the slave calls
 <pre>message_from_master = spi_read(123);</pre>

@@ Line 68: / Line 68: @@
 == Code ==
-SPI communication on the PIC is accomplished via the functions spi_read and spi_write, which read and write one byte at a time over SPI. In the example below, the master reads an 8-bit value from the analog-to-digital converter and sends it to the slave via SPI. The slave reads the value and displays it on an LCD display.
+SPI communication on the PIC is accomplished via the functions spi_read() and spi_write(), which read and write one byte at a time over SPI. In the example below, the master reads an 8-bit value from the analog-to-digital converter and sends it to the slave via SPI. The slave reads the value and displays it on an LCD display.
 <b>SPI Modes</b>
@@ Line 187: / Line 187: @@
+<b>Two-way communication</b>
+Though not shown above, it is possible to use SPI for two-way communication between master and slave. If the slave calls
+<pre>message_from_master = spi_read(123);</pre>
+It will wait for the master to send some clock pulses; when the clock pulses come the slave will simultaneously read the message being sent by the master and send its own message (the number 123). The master, for its part, will call
+<pre>message_from_slave = spi_read(22);</pre>
+This will cause the master to send the number 22 to the slave and simultaneously read the message sent back by the slave (in this case the number 123). So the master and slave have exchanged one byte of information. If you are the master and you just want to get a byte from the slave without sending one, just send a junk value, which the slave will discard:
+<pre>message_from_slave = spi_read(0);</pre>
+<b>Useful resources</b>
 The CCS user manual