Keil Logo

C51: OPTIMUM CODE FOR BIT TO BYTE CONVERSION


Information in this article applies to:

  • C51 All Versions
  • CX51 All Versions
  • C251 All Versions

QUESTION

I have a very time-critical routine that accesses an SPI Bus. It needs to shift in 8 bits from one single pin to a register. Normally this would be done in a loop, but I must optimize it to consume minimum time.

The construction below works OK, but still does not give me the performance that I would like to have. Is there a better way to code this?

  unsigned char Read_SPI_Byte (void)  {
  unsigned char result;

  result = 0;
  SCLK = 0;
  if (MISO) result |= 0x80;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x40;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x20;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x10;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x08;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x04;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x02;
  SCLK = 1;
  SCLK = 0;
  if (MISO) result |= 0x01;
  SCLK = 1;
  return (result);
}

ANSWER

Yes. You may use bdata variables (or in this case even misuse the SFR register B to reduce the data overhead. The following code will give you the optimum performance:

sfr B=0xF0;     // usually this is included in the CPU register header file
sbit B0=B^0;    // define indivitual bits in the B register
sbit B1=B^1;
sbit B2=B^2;
sbit B3=B^3;
sbit B4=B^4;
sbit B5=B^5;
sbit B6=B^6;
sbit B7=B^7;

unsigned char Read_SPI_Byte (void)  {
  SCLK = 0;
  B7 = MISO;    // NOTE: SFR B can be used, since the code does not
  SCLK = 1;     //       affect other statements. However, you
  SCLK = 0;     //       should carefully check the assembler code
  B6 = MISO;    //       listing when you are using this CPU register.
  SCLK = 1;     //       Typically the B register is used for multiplication,
  SCLK = 0;     //       division, pointer access, and math routines.
  B5 = MISO;
  SCLK = 1;
  SCLK = 0;
  B4 = MISO;
  SCLK = 1;
  SCLK = 0;
  B3 = MISO;
  SCLK = 1;
  SCLK = 0;
  B2 = MISO;
  SCLK = 1;
  SCLK = 0;
  B1 = MISO;
  SCLK = 1;
  SCLK = 0;
  B0 = MISO;
  SCLK = 1;
  return (B);
}

MORE INFORMATION

SEE ALSO

Last Reviewed: Friday, July 15, 2005


Did this article provide the answer you needed?
 
Yes
No
Not Sure
 
  Arm logo
Important information

This site uses cookies to store information on your computer. By continuing to use our site, you consent to our cookies.

Change Settings

Privacy Policy Update

Arm’s Privacy Policy has been updated. By continuing to use our site, you consent to Arm’s Privacy Policy. Please review our Privacy Policy to learn more about our collection, use and transfers
of your data.