Post by thread:[PIC] Software SPI

Hi to all, Sorry guys I forgot even say thank you all for my previews post. I am having problem with software spi I put it together and I am wondering if it looks ok. To my understanding it should work. void write_umFPU_byte(BYTE data_byte, BYTE number_of_bits) { BYTE i; // delay_us(10); for(i=0; i<number_of_bits; ++i) { output_low(umFPU_CLK); // if((data_byte & 0x80)==0) // MSB output_low(umFPU_DIN); // else output_high(umFPU_DIN); // data_byte=data_byte<<1; // delay_us(30); output_high(umFPU_CLK); // delay_us(30); } } this is going to shift MSB first. any idea ? do you see any problem with the software ? thanks Andre

In my most recent software SPI, I had a bitmask that I initialized at 0x80. I then anded it with the data byte, set the MOSI high or low as required, then shifted the bitmask right one bit. This was looped until the bitmask was shifted down to nothing. Something like this: void SendByte(uint8_t data){ uinit8_t BitMask=0x80 while(BitMask){ SCLK=0; if(BitMask & data) MOSI=1; else MOSI=0; SCLK=1; BitMask>>1; } } Harold {Quote hidden}> Hi to all, > > Sorry guys I forgot even say thank you all for my previews post. > I am having problem with software spi I put it together and I am > wondering if it looks ok. To my understanding it should work. > > void write_umFPU_byte(BYTE data_byte, BYTE number_of_bits) > { > BYTE i; > // delay_us(10); > for(i=0; i<number_of_bits; ++i) > { > output_low(umFPU_CLK); // > if((data_byte & 0x80)==0) // MSB > output_low(umFPU_DIN); // > else > output_high(umFPU_DIN); // > data_byte=data_byte<<1; // > > delay_us(30); > output_high(umFPU_CLK); // > delay_us(30); > } > } > > this is going to shift MSB first. > any idea ? > do you see any problem with the software ? > > > thanks > Andre > > -

{Quote hidden}>-----Original Message----- >From: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@spam@mit.edu] >On Behalf Of Andre Abelian >Sent: 11 July 2007 23:57 >To: Microcontroller discussion list - Public. >Subject: [PIC] Software SPI > > >Hi to all, > >Sorry guys I forgot even say thank you all for my previews >post. I am having problem with software spi I put it together >and I am wondering if it looks ok. To my understanding it should work. > >void write_umFPU_byte(BYTE data_byte, BYTE number_of_bits) > { > BYTE i; >// delay_us(10); > for(i=0; i<number_of_bits; ++i) > { > output_low(umFPU_CLK); // > if((data_byte & 0x80)==0) // MSB > output_low(umFPU_DIN); // > else > output_high(umFPU_DIN); // > data_byte=data_byte<<1; // > > delay_us(30); > output_high(umFPU_CLK); // > delay_us(30); > } > } > >this is going to shift MSB first. >any idea ? >do you see any problem with the software ? It will probably work ok, but a few comments: 1) Do you really need such slow timing, i.e. have you made it deliberately slow for EMI compliance, or do you have a very slow slave device? 2) Note that SPI devices work in one of four modes, which relate to the relationship between clock and data, i.e. the data can be clocked in on a rising or falling clock edge, and it can be a leading or trailing clock edge. The PIC datasheets show the differences if you look under the MSSP peripheral. The datasheet for your SPI peripheral should show a timing diagram from which which the appropriate mode can be worked out. Just as a note on your loop, it's usualy better to decrement counters on a PIC as it often results in smaller and faster code e.g. while( number_of_bits-- ) { /* send one bit */ } This allows the compiler to use a desfsz instruction instead of having to increment the counter and then compare against a non-zero value. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================