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SuperDave
Joined: 22 May 2008
Posts: 63
Location: Madison, TN
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| spi_xfer with 7 bytes |
 Posted: Wed Mar 24, 2010 10:15 am |
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I need to send 7 bytes via spi_xfer.
I can do this (pseudo code)
define array of 7 bytes
define stream without enable and 8 bits max transfer
define enable
set enable
for next 7 bytes from array
unset enable
That works but is not elegant.
Question
Can I define the stream with enable and 56 bits max transfer and then
spi_xfer(stream, array);
e.g. is an array considered data within the context of an spi_xfer? |
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MikeW
Joined: 15 Sep 2003
Posts: 184
Location: Warrington UK
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| Posted: Wed Mar 24, 2010 10:34 am |
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I am pretty sure you can do it with the software SPI, not sure about the hardware SPI though..
I use
| Code: |
#use spi(Master, Mode=0, DI=Touch_Screen_SPI_MISO_IN, DO=Touch_Screen_SPI_MOSI_OUT, CLK=Touch_Screen_SPI_SCLK, ENABLE=Touch_Screen_SPI_CS, ENABLE_ACTIVE=0,BITS=24, MSB_FIRST)
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to transfer 24 bits at a time
then
| Code: |
void Check_Touchscreen()
{
int32 spi_buf[3];
spi_buf[0] = spi_xfer(TC_CR_Y);
spi_buf[1] = spi_xfer(TC_CR_X);
etc
etc
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Ttelmah
Joined: 11 Mar 2010
Posts: 19549
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| Posted: Wed Mar 24, 2010 10:46 am |
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However there is nothing 'non elegant' about the original solution. Just generate your own wrapper. So something like:
| Code: |
send_spi(int8 * data, int8 num_bytes) {
int8 ctr=0;
output_low(CS); //change to suit your enable polarity and pin
do {
spi_read(data[ctr++]);
} while (ctr<num_bytes);
output_high(CS); //again change
}
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Using spi_read, ensures that the byte has _finished_ clocking, before the function returns. On some of the later chips, the write returns as soon as the byte is transferred to the output buffer.
This is after all, exactly what any internal 'CCS' function is going to have to do. You would obviously need to add stream identifiers if these are being used, and I have generated the function, so it can send any number of bytes (up to 255).
Best Wishes |
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SuperDave
Joined: 22 May 2008
Posts: 63
Location: Madison, TN
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| Posted: Thu Mar 25, 2010 5:52 am |
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| Code: | int32 spi_buf[3];
spi_buf[0] = spi_xfer(TC_CR_Y);
spi_buf[1] = spi_xfer(TC_CR_X);
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This is what I do elsewhere when I need 24 bits using an int32. But it has the problem that the enable line drops between spi_xfers, meaning that since there is no int64 it won't work for 56 bits. (Persons reading this at some future time should note that since the transfer is MSB first, the least significant byte is undefined, never sent and never read.)
| Code: | do {
spi_read(data[ctr++]);
} while (ctr<num_bytes);
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Indeed works but is 'inelegant' in the sense that spi is being done differently it two areas of the same code. Not fatal, just thinking of the guy in the future that has to maintain this.
It would be nicer if spi_xfer(stream,DATA) could be interpreted as being valid to mean spi_xfer(stream,ARRAY). That's really the central question. I was hoping there was someone out there who had made that work successfully. Looks like I'll have to experiment.
Thanks for the help.
Dave |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19549
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| Posted: Thu Mar 25, 2010 6:22 am |
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Again wrapper it yourself.
Remember that the compiler now supports function overloading, so just make the wrapper function, apply if spi_xfer, is called with a pointer.
Use the sizeof operator to get the size of the object, and "Robert's your mother's brother".
Serioulsy, the more that is done 'under the hood' by CCS, the more likely things are to have problems with different versions. Personally, I wish that CCS just made the compiler 'invisible', and all the standard functions were supplied in libraries, rather than hidden from the user.
As for using different ways of writing, this is what documentation is for. A simple comment on the line like:
//Writing, using the read function, to ensure data _has_ been clocked out.
tells a future devloper what is being done.
Best Wishes |
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