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FRAM FM25L256 read/write problem

 
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microWip



Joined: 19 May 2007
Posts: 2

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FRAM FM25L256 read/write problem
PostPosted: Sat May 19, 2007 4:03 pm     Reply with quote
Hello

I'm using a PIC 18F6722 to read and write to a FRAM chip through SPI.

I use this code:

Code:

// OP codes
#define WREN  0x06
#define WRSR 0x01
#define READ  0x03
#define WRITE 0x02

//variabels
int8 temp = 0x02;


void init_fram() {
   setup_spi(SPI_MASTER | SPI_XMIT_L_TO_H | SPI_CLK_DIV_4);
   output_high(cs_fram);
   delay_ms(11);
   
   output_low(cs_fram);
   spi_write(WREN);
   output_high(cs_fram);

   output_low(cs_fram);
   spi_write(WRSR);
   spi_write(0x80); // sets bank protect bits to 0, and enables external WP line
   output_high(cs_fram);
   
   
}

void write_fram(int16 address, int8 data) {
   output_low(cs_fram);
   spi_write(WREN);
   output_high(cs_fram);

   output_low(cs_fram);
   spi_write(WRITE);
   spi_write(address>>8);  //send the high int8 of the address
   spi_write(address);
   spi_write(0x33);
   output_high(cs_fram);
   
   delay_ms(11);
}

int8 read_fram(int16 address) {
   int8 data;

   output_low(cs_fram);
   spi_write(READ);
   spi_write(address>>8);  //send the high int8 of the address
   spi_write(address);
   data=spi_read(0);
   output_high(cs_fram);

   return(data);
}



I write a byte to an address an try to read afterwards, but all I get is 0xFF. I thought that 0xFF is the default value of a memory entry so I presume my read code is correct.

Code:

void main() {

   init_fram();
   
   
   while(true)
   {
      if(!input(knop1))
      {
         temp = read_fram(0x0001);
         putC(temp);
         delay_ms(500);
      }
      if(!input(knop2))
      {
         write_fram(0x0001, 0x22);
         output_low(LED_ON);
         delay_ms(500);
         output_high(LED_ON);
      }
   }
}


Does anyone sees errors in my code? Please help...

Kinde regards
Ttelmah
Guest
PostPosted: Sun May 20, 2007 2:44 am     Reply with quote
First, I'd suggest using the SPI 'mode numbers', published here in the past. I think by Ckielstra?. These make it much simpler to setup the registers:
Code:

#define SPI_MODE_0_0 (SPI_L_TO_H | SPI_XMIT_L_TO_H)
#define SPI_MODE_0_1 (SPI_L_TO_H)
#define SPI_MODE_1_0 (SPI_H_TO_L)
#define SPI_MODE_1_1 (SPI_H_TO_L | SPI_XMIT_L_TO_H)

At the front of the data sheet, Ramtron say this chip supports the first, and last of these. However all the diagrams show it using the first. This should be what your current setup gives, but worth being sure.
Second comment. Are you running in standard IO mode, or fast IO mode?. If the latter, then I would suggest adding 1 clock cycle of delay on the strobe of the CS line (worth trying anyway). The PIC, samples the IO pin, at the start of the instruction, and updates it near the end of the instruction. Hence a pair of writes, one after the other, with no delay between can (depending on bus capacitance), result in a pin that doesn't update...
I presume WP, is wired high?. You make no mention of how it is wired?.
You don't need the delay after the write to the fram. Writes on these chips are basically instantaneous.
Is the 'hold' pin also wired high?.

Best Wishes
microWip



Joined: 19 May 2007
Posts: 2

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PostPosted: Sun May 20, 2007 3:16 am     Reply with quote
Thank you very much !!! Exclamation

I followed al your suggestions and it worked:

I added in the inititalisation:
Code:
setup_spi(SPI_MASTER | SPI_MODE_1_1  | SPI_CLK_DIV_4);   
   output_high(hold_fram);  //chip select
   output_high(wp_fram);



In the datasheet is mentioned dat the clocking of data in is on the falling clock edge, clocking data out on the rising clock edge, so I use SPI_MODE_1_1

Between chip select toggles:
Code:
 output_high(cs_fram);
   delay_cycles(1);
   output_low(cs_fram);


This changes made everything work
Very Happy

One last question: you talk about fast I/O mode...were can I see that I'm using that mode?

kind regards
Ttelmah
Guest
PostPosted: Sun May 20, 2007 4:21 am     Reply with quote
If you are not setting it, you are not using it.
The setup, would be:
#use fast_io(A)

and similar lines for the other ports, in the 'header' for your program, and then you would need to add a TRIS statement, setting the pins as inputs/outputs.
Basically, without this, an instruction like:

output_high(PIN_A0);

gets translated into setting the TRIS bit for this pin to 0, and then outputting the bit. Similarly, an input instruction, changes the tris to '1', and reads the bit. Hence every single I/O instruction, gets an extra delay added, while the TRIS is updated (even if it is not changing...).
Using fast_io, it becomes your responsibility to setup the tris, but unless you are using changes in I/O direction on a pin, this can be done just once, speeding up every I/O operation.

Best Wishes
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