INFICON Spot CDS550D - Manual (Page 8)

Research & Development
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.1 Page 8 of 14
Christian Berg, T based on Felix Mullis, TL Created 11/10/2014 2:36:00 PM
3.2 Temperature
The SPI communication string for reading the temperature value consists of total 4 bytes as follows:
0x4D – 0x00 – 0x00 – 0x00, only the first byte is relevant, the other three must be sent but the content
of them doesn’t matter.
Read Temperature Op-Code:
Byte_3 Byte_2 Byte_1 Byte_0
0 1 0 0 1 1 0 1 don’t’care
The result (u_temp) 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_temp) 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 temperature is defined as k * u_temp, where k is defined as a Calibration Constant.
(Typically 25°C).
T = k * u_temp
(24 bit read result)
; k = 25°C
(Calibration Temperature)
u_temp is calculated by dividing the 24-bit result by 2
21
. 2
21
equal 2’097’152.
Examples, assuming k = 25°C:
•
T ≥ 100°C: 24-bit result = 8’388’607 (0x7FFFFF)
•
T = 50°C: 24-bit result = 4’194’304 (0x400000)
•
T = 25°C: 24-bit result = 2’097’152 (0x200000)
•
T = 0°C: 24-bit result = 0 (0x000000)
•
T = -25°C: 24-bit result = -2’097’152 (0xE00000)