SPI slave on ATmega162: transferring data

fine,

What data is received, what do you see on the screen?

Meindert

I have a feeling that the problem is that you aren't thinking how a slave should react. You are still thinking as if it is a master. A slave only responds to a request from a master. A slave does not initiate transfers.

If you really want to see just your data come across, you need to setup the first data byte on initialization. When the master starts clocking, so will the slave. I don't know what triggers your SIG_SPI, maybe the toggling of SS? If so, then you have only the amount of time from the changing of SS to the Master starting to clock in order to get your data into the data register if it wasn't put there previously. I'm not sure what happens if you change the data register while the shifting is happening. Maybe the 162 will save it away in a buffer to be written after the transfer or maybe it won't allow the change, or maybe it will allow the change and the master will get some bits from the old data and some from the new data.

And remember that the data register gets overwritten by watever the master sent.

What you probably want to do in this specific case is:

on slave init: setup for SPI Slave, then write first byte of data into SPDR.

after transmission complete: get next data byte and write into SPDR for next transmission.

Quite frankly, I also don't like this. You change the global variable "data" and then restore it. Why not just change the temporary "t" and do away with the restore? Only change a global if that side effect is required.

Hello, Gary! You wrote on 09 Dec 2004 11:40:57 GMT:

GK> If you really want to see just your data come across, you need to setup GK> the first data byte on initialization. When the master starts clocking, Do you mean, I should write byte into SPDR at my SPI_SlaveInit function? If so, should I also wait for transfer complete or at that stage it's quite enought just store value in the SPDR register? But if slave doesn't initiate a transfer it can silently wait for a moment when master starts transfer. I don't quite understand why should I write byte at slave init? GK> so will the slave. I don't know what triggers your SIG_SPI, maybe the GK> toggling of SS? If so, then you have only the amount of time from the As I understood datasheet, interrupt will be invoked as soon as transfer of byte will be completed. GK> changing of SS to the Master starting to clock in order to get your GK> data into the data register if it wasn't put there previously. I'm not

[skip]

GK> What you probably want to do in this specific case is:

GK> on slave init: setup for SPI Slave, then write first byte of data into GK> SPDR.

GK> after transmission complete: get next data byte and write into SPDR for GK> next transmission. ok, I'll follow your advice ??>> char *t; ??>>

??>> t = data; ??>> while ( *data != 0 ) { ??>> SPI_SendByte((char *)data); ??>> data++; ??>> } ??>> data = t;

GK> Quite frankly, I also don't like this. You change the global variable "data" GK> and then restore it. Why not just change the temporary "t" and do away with the GK> restore? Only change a global if that side effect is required. In this case, may be it's better to declare 'char *t' as global volatile variable and use it, or it's rather useless ?

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

Hello, Meindert! You wrote on Thu, 9 Dec 2004 11:55:22 +0100:

??>> After receiving data from me, master is logging them onto the screen, ??>> so I can watch what's going on. Indeed it happenes, data got by master ??>> are strange and not what I sent.

MS> What data is received, what do you see on the screen? on the screen I see like this:

0x01 0x02 0x01 0x00 0x04 0x03 0x02 0x01

ans so on

and from time to time the order of data is changed.

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

Hello, Gary! You wrote on 09 Dec 2004 11:40:57 GMT:

GK> And remember that the data register gets overwritten by watever the GK> master sent.

GK> What you probably want to do in this specific case is:

GK> on slave init: setup for SPI Slave, then write first byte of data into GK> SPDR.

GK> after transmission complete: get next data byte and write into SPDR for GK> next transmission. I don't quite understand if I should check SPIF flag in case of interrupt driven code?

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

Below, you pass a pointer to a character:

And here, you pass a character to SPI_SendByte, while it expects a pointer... Can't be right.

Meindert

Hello, Meindert! You wrote on Thu, 9 Dec 2004 13:58:24 +0100:

MS> Below, you pass a pointer to a character: I changed the code and simplified a little, now I don't use pointers and send only one byte at all:

... char *t_data = "1";

void SPI_SendByte(char data) { SPDR = data; while ( !(SPSR & (1

The problem with the code above (I assume it is the code for the master) is that you call spi_sendbyte from within the interrupt handler. Spi_sendbyte waits for the SPIF flag. The moment this flag is set, another interrupt is issued while you are still inside spi_sendbyte. You can safely remove the while loop in spi_sendbyte because you only call it once at the start of the communication. After that it is called from the interrupt handler only when the shifter is empty. So, no need to wait for the SPIF flag. And when the while loop is removed, you'll that the only line left in spi_sendbyte can be put in the interrupt handler to save a pair of unnecessary call/return.

Meindert

Hello, Meindert! You wrote on Thu, 9 Dec 2004 14:40:44 +0100:

MS> The problem with the code above (I assume it is the code for the MS> master) is that you call spi_sendbyte from within the interrupt this is code for slave MS> handler. Spi_sendbyte waits for the SPIF flag. The moment this flag is MS> set, another interrupt is issued while you are still inside MS> spi_sendbyte. You can safely remove the while loop in spi_sendbyte Alright, I understand you. Here is changed version, I hope is correct :) MS> because you only call it once at the start of the communication. After MS> that it is called from the interrupt handler only when the shifter is MS> empty. So, no need to wait for the SPIF flag. And when the while loop MS> is removed, you'll that the only line left in spi_sendbyte can be put MS> in the interrupt handler to save a pair of unnecessary call/return.

char *t_data = "1"; ...

SIGNAL(SIG_SPI) { SPDR = *t_data; LED0_blink(); // debugging feature }

void SPI_SlaveInit(void) { DDRB = (1

Almost. You do not detect whether you are at the end of the array or ready sending data.

Meindert

What is the frequency of the SCK signal from the master? Sometimes simulators will respond to clock signals too fast for the actual hardware.

Most of the SPI systems I've worked with use a separate signal as a chip select. An SPI slave device will generally require a low-going chip select from the master device to inform it that a transfer is pending. In your case, you could use that incoming low-going CS signal to start the transfers. However, you will need to ensure that there is enough time between the CS signal and the start of the transfer. As noted in another post, you may have to 'pre load' the SPI data register so that the hardware can send the byte as soon as the master starts SCK. The CS interrupt service routine then simply loads the NEXT byte when the first transfer is done and polls the SPIF bit to determine when to load subsequent bytes.

If you rely on an SPIF interrupt, you need VERY fast interrupt service to get the next byte into the output register before clocking starts, if the master is requesting bytes one after the other in continuous fashion.

Mark Borgerson

And if the LED0_blink() takes longer than 8 SCK cycles to execute, you're going to have problems with continuous data. If LED0_Blink() simply sets a flag so that the main routine can blink the LED, you may be OK.

You haven't said what SCK rate is being used by the master. If the slave is being implemented on an 8MHz machine, I suspect it may not like SCK signals in excess of 2MHz. If the SCK is

2Mhz, You don't have a lot of machine cycles to respond to the interrupts.

I would suggest starting with a clock rate in the 100KHz region, with perhaps a millisecond between transfers at the host end. When that works, you can start increasing the SCK rate. One of the nice things about being an SPI slave is that you don't really have to worry about slow clock rates.

Mark Borgerson

... snip ...

It is more efficient to do other things while the transmitter becomes ready, i.e. use:

while (!(SPSR % (1

Hello, Meindert! You wrote on Thu, 9 Dec 2004 15:27:20 +0100:

??>>

MS> Almost. You do not detect whether you are at the end of the array or MS> ready sending data. I don't have any array, I'm sending just 1 byte.

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

Hello, Mark! You wrote on Thu, 09 Dec 2004 15:35:51 GMT:

MB> And if the LED0_blink() takes longer than 8 SCK cycles to execute, MB> you're going to have problems with continuous data. If LED0_Blink() MB> simply sets a flag so that the main routine can blink the LED, MB> you may be OK. Yeah,, may be that's the main issue. My LED0_blink() function is pretty dumb and I'm not sure it's fast enought to jump out of ISR:

void LED0_blink(void) { uint8_t i=0; DDRC = 0x01;

// flash LED (LED is inverse) PORTC &= ~(1 to the interrupts. SCK rate of master is 500KHz MB> I would suggest starting with a clock rate in the 100KHz region, MB> with perhaps a millisecond between transfers at the host end. MB> When that works, you can start increasing the SCK rate. MB> One of the nice things about being an SPI slave is that you MB> don't really have to worry about slow clock rates.

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

Always do as little as possible in your interrupt service routines.

Yes, but you're sending it over and over again until you hit the reset button.

Meindert

Hello, Meindert! You wrote on Fri, 10 Dec 2004 07:02:42 +0100:

MS>>> Almost. You do not detect whether you are at the end of the array or MS>>> ready sending data. ??>> I don't have any array, I'm sending just 1 byte.

MS> Yes, but you're sending it over and over again until you hit the reset MS> button. Do you mean, some flag indicating the status of process should be included in code?

With best regards, Roman Mashak. E-mail: snipped-for-privacy@tusur.ru

or

reset

Well, it depends on what you want to achieve. Your original code sent out a string, there you have to test for the end of the string. Later, you changed your code to send only one byte, to see what went wrong. But that code keeps sending indefinately because of the interrupt routine that gets called after you send the first and only byte 'manually'. Oh, and as other already pointed out: in an interrupt routine that runs very frequently (do you actually know how often it is executed? You should!), you cannot blink a LED and then wait in a nested loop. During that loop, the interrupt routine might get called another few 10-100 thousand times but not executed because the processor inhibits interrupts unless you enable them explicitly within the interrupt handler. If all this sounds like chinese to you, with all respect try something more easy to achive until you thoroughly understand what the processor is doing during interrupts.

Meindert