Difficulties with twos complement? Perhaps its not being able to use the 19 bit and the ~~ command.

Zap-o

While asking for data that resides in a register aboard the BMP180 digital pressure IC. I do the following...

Take two BYTES out of two registers (BMP180)
Combine them into one LONG labeled AC1
Send this data to the CHECK_SIGN(AC1) for twos complement.

Pub Read_Constants

  AC1 := i2c.rd_BYTE(BMP180,$AA) << 8 | i2c.rd_BYTE(BMP180,$AB) 'read one byte of data each, from two registers AC1 is a long. 
  AC1 := check_sign(AC1)  

and..

Pub Check_sign(in) : out 

  out := ~~in

So far, in the codes shown above everything works as expected, using two bytes, then combining to make a long. Unfortunately the BMP180 has the need to take three bytes and combine them into a long yet still needing to accomplish the post extend operation, I ask how?

The ~~X portion of the expression extends the sign bit from bit 15 all the way up to bit 31

My trouble is when I need to start at bit 19 instead of the 15th bit.

  UP := i2c.rd_BYTE(BMP180,$F6) << 16 | i2c.rd_BYTE(BMP180,$F7) << 8 | i2c.rd_BYTE(BMP180,$F8)
  Up := Check_sign(Up)

I Hope this I made this clear enough.

and, Thanks in advance.

JasonDorie

Simple - Use a normal shift to move everything up by (31 - 19) = 12 bits to get bit 19 to the highest bit position.

Now shift back down using arithmetic shift to replicate the high bit.

Pub Check_sign(in) : out 
  out := (in << 12) ~> 12

geo_leeman

There is also a recent discussion of this sensor with links to a basic driver that may end up reducing the code you need to write.

Zap-o

Problem solved as I see in GetHub the code and compare it to mine. Thanks for the help.