INFICON Spot CDS550D - Manual (Page 7)

Research & Development
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.1 Page 7 of 14
Christian Berg, T based on Felix Mullis, TL Created 11/10/2014 2:36:00 PM
3 Read Results
The sensor provides 3 values:
•
Pressure
•
Temperature
•
Status
They can be read by a 4 byte SPI-Code as follows:
3.1 Pressure
The SPI communication string for reading the pressure value consists of total 4 bytes as follows:
0x41 – 0x00 – 0x00 – 0x00, only the first byte is relevant, the other three must be sent but the content
of them doesn’t matter.
Read Pressure Op-Code:
Byte_3 Byte_2 Byte_1 Byte_0
0 1 0 0 0 0 0 1 don’t care
The result (u_press) is a fixed-point number in two’s complement format with a scaling factor of 2
21
as
follows:
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Byte_2 Byte_1 Byte_0
S Integer Fractional (21Bits)
The result (u_press) is a fixed-point number in two’s complement format with a scaling factor of 2
21
,
consisting of the sign-bit S, 2 integer and 21 fractional bits. The fourth byte (Byte_3), that’s also re-
ceived, must be ignored.
The resulting pressure is defined as Full_scale * u_press.
p = F.S.R. * u_press
(24 bit read result)
u_press is calculated by dividing the 24-bit result by 2
21
. 2
21
equals 2’097’152.
Examples:
•
Fullscale (F.S.R.): 24-bit result = 2’097’152 (0x200000)
•
Half F.S.R.: 24-bit result = 1’048’576 (0x100000)
•
smallest pos. val: 24-bit result = 1 (0x000001) equals to 0.00000047683 * F.S.R.
•
Zero Pressure: 24-bit result = 0 (0x000000)
•
smallest neg. val: 24-bit result = -1 (0xFFFFFF) equals to -0.00000047683 * F.S.R.
•
neg. Half F.S.R.: 24-bit result = -1’048’576 (0xF00000)
•
neg. F.S.R. 24-bit result = -2’097’152 (0xE00000)