Research & Development
SPI-Interface
Specifications
for SPOT
CDS500D & CDS550D
OEM
Capacitance Diaphragm
Sensors
V 1.0
January 17, 2014
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
Page 1 of 14
Created 1/17/2014 5:11:00 PM
Research & Development
1 Table of Contents
1 Table of Contents ............................................................................................................. 2
2 Hardware Concept ............................................................................................................ 3
2.1 Hardware Block Diagram ............................................................................................................... 3
2.2 Interface Connector and Cable ...................................................................................................... 4
2.3 Grounding Concept ........................................................................................................................ 4
2.4 Timing Specifications ..................................................................................................................... 4
2.5 Interface Hardware Specifications ................................................................................................. 5
2.5.1 Power Supply .......................................................................................................................... 5
2.5.2 Recommended operating conditions....................................................................................... 5
2.5.3 SPI Interface............................................................................................................................ 6
3 Read Results..................................................................................................................... 7
3.1 Pressure ......................................................................................................................................... 7
3.2 Temperature .................................................................................................................................. 8
3.3 Status and Error ............................................................................................................................. 9
3.4 Power-up reset ............................................................................................................................... 9
4 History ............................................................................................................................... 9
5 Appendix ..........................................................................................................................10
5.1 Grounding Concepts .................................................................................................................... 10
5.2 SPI Timing ................................................................................................................................... 11
5.2.1 Introduction ............................................................................................................................ 11
5.2.2 Timing .................................................................................................................................... 11
5.2.3 Total Measurement Cycle ..................................................................................................... 12
5.2.4 Ready-signal ......................................................................................................................... 12
5.2.5 Single Measurement Cycle ................................................................................................... 13
5.2.6 Readout Time Window .......................................................................................................... 13
5.2.7 Conclusion ............................................................................................................................. 14
5.2.8 FAQ’s, Comments ................................................................................................................. 14
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
2 Hardware Concept
This section describes how two or more SPOT CDS5xxD slaves must be physically connected together with a master. When doing so, the master will be able to receive pressure and temperature data
from each SPOT CDS5xxD slave.
The data exchange link consists of a simple SPI-Interface; therefore only three lines and a chip select
signal are necessary. The additional RDY/ lines signal the end of a measurement cycle. It must be
used in noise critical applications. There it’s forbidden to read data from the chip during measurement
cycles. (Crosstalk from the SPI-interface to the measurement data)
2.1 Hardware Block Diagram
Figure 2.1-1: SPI Block Diagram
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
Page 3 of 14
Created 1/17/2014 5:11:00 PM
Research & Development
2.2 Interface Connector and Cable
Connector Type: JST 1mm 10 pol.
pin
Signal
Description
1
2
3
4
5
6
7
8
9
10
+5V
GND
Clk
SS/
MISO
MOSI
RDY/
C1/C0
R0/Rref
Vpp_OTP
Power Supply
Supply Common
SPI Clock
SPI Slave Select (low true)
SPI Master Input Slave Output
SPI Master Output Slave Input
Ready Signal (new measurement value available)
Internal use only, must be left open
Internal use only, must be left open
Internal use only, must be left open
Direction
(SPOT CDS5xxD-side)
input
input
input
input (low true)
output (tristate)
input
output (low true)
output
output
input
Table 1: pin assignment SPI connector
Cable length: max. 15 cm
2.3 Grounding Concept
The metallic sensor housing (flange) acts as a shielding for the very sensitive electronic measurement
frontend. In order to achieve the specified noise specifications, the sensor flange must be connected
to GND (Supply Common). The impedance of such a connection should be as low as possible. Therefore it is realized directly on the PCB at the frontend of the electronics. This solution guarantees lowest
noise and simultaneously maximizes EMC robustness.
2.4 Timing Specifications
The SPI Timing is described in detail in chapter 5.2 (SPI Timing) in the Appendix.
Signal
T_tot_cycle
T_sgl_cycle
T_rd_out
Description
Total Measurement Cycle Time = Data Cycle Time
Single Measurement Cycle Time
Readout Time
MIN
600
500
-
TYP
700
600
-
MAX
800
700
100
UNIT
µs
µs
µs
Table 2: SPI Readout Timing
For a description of the above used expressions refer to chapter 5.2 (SPI Timing) in the Appendix.
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
2.5 Interface Hardware Specifications
2.5.1 Power Supply
Signal
Vcc
Ripple
Icc
Description
Supply voltage
Supply voltage ripple and noise (BW = 20MHz)
Supply current
MIN
4.75
TYP
5.0
1
MAX
5.25
50
5
UNIT
VDC
mVpk-pk
mA
2.5.2 Recommended operating conditions
The input lines “Clk”, “SS/” and “MOSI” and the output line “MISO” are buffered with a SN74ABT125.
The output line “RDY/” is driven by a SN74AHCT1G125
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
2.5.3 SPI Interface
2.5.3.1 SPI Parameters:
Parameter
CPOL
CPHA
Mode
DORD
Description
Clock Polarity
Clock Phase
SPI Mode
Bit sequence order
Setting
0, Clock Idle low
1, Transmit data on leading edge, read data on falling edge
1 (CPOL = 0, CPHA = 1)
0, MSB first
2.5.3.2 SPI Write
2.5.3.3 SPI Read
2.5.3.4 SPI Timing
Name
Clock frequency
Clock pulse high-state
Clock pulse low-state
SS/ (SSN) to valid latch
SS/ (SSN) minimal pulse length between write cycles
Data setup time
Data hold time
Data valid after clock
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
Symbol
fSPI-bus
tpwh
tpwl
tsussn
tpwssn
tsud
thd
tvd
Value
17
30
30
8
30
6
4
26
Unit
MHz
ns
ns
ns
ns
ns
ns
ns
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Research & Development
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
0 1 0 0 0 0 0 1
Byte_2
Byte_1
don’t care
Byte_0
21
The result (u_press) is a fixed-point number in two’s complement format with a scaling factor of 2 as
follows:
23
22
21
20
19
18
Byte_2
S
Integer
17
16
15
14
13
12
11
10
9
8
7
6
Byte_1
Fractional (21Bits)
5
4
3
2
1
0
Byte_0
21
The result (u_press) is a fixed-point number in two’s complement format with a scaling factor of 2 ,
consisting of the sign-bit S, 2 integer and 21 fractional bits. The fourth byte (Byte_3), that’s also received, must be ignored.
The resulting pressure is defined as Full_scale * u_press.
p = F.S.R. * u_press (24 bit read result)
21
21
u_press is calculated by dividing the 24-bit result by 2 . 2 equals 2’097’152.
Examples:
• Fullscale (F.S.R.):
• Half F.S.R.:
• smallest pos. val:
• Zero Pressure:
• smallest neg. val:
• neg. Half F.S.R.:
• neg. F.S.R.
24-bit result = 2’097’152 (0x200000)
24-bit result = 1’048’576 (0x100000)
24-bit result = 1 (0x000001) equals to 0.00000047683 * F.S.R.
24-bit result = 0 (0x000000)
24-bit result = -1 (0xFFFFFF) equals to -0.00000047683 * F.S.R.
24-bit result = -1’048’576 (0xF00000)
24-bit result = -2’097’152 (0xE00000)
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
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
0 1 0 0 1 1 0 1
Byte_2
Byte_1
don’t’care
Byte_0
21
The result (u_temp) is a fixed-point number in two’s complement format with a scaling factor of 2 as
follows:
23
22
21
20
19
Byte_2
S
Integer
18
17
16
15
14
13
12
11
10
9
8
7
Byte_1
Fractional (21Bits)
6
5
4
3
2
1
0
Byte_0
21
The result (u_temp) is a fixed-point number in two’s complement format with a scaling factor of 2 ,
consisting of the sign-bit S, 2 integer and 21 fractional bits. The fourth byte (Byte_3), that’s also received, 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
21
(Calibration Temperature)
21
u_temp is calculated by dividing the 24-bit result by 2 . 2 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)
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
Page 8 of 14
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Research & Development
3.3 Status and Error
The SPI communication string for reading the status value consists of total 4 bytes as follows:
0x48 – 0x00 – 0x00 – 0x00, only the first byte is relevant, the other three must be sent but the content
of them doesn’t matter.
Read Status Op-Code:
Byte_3
0 1 0 0 1 0 0
Status result:
Byte_3
don’t’ care
Bit
23
13
8
7
6
5
3
Byte_2
Byte_1
don’t care
Byte_0
Byte_2
Byte_1
24-Bit Status value (Bit 23..0)
Byte_0
0
Description
SPI communication took place during measurement
Pressure error
port 3 error
port 2 error
port 1 error
port 0 error
Temperature error
All other bits not mentioned here must be ignored.
3.4 Power-up reset
After powering up the device, it must be reset by sending the reset command to the device. The reset
command consists of the single byte 0x88h. It is highly recommended to send the reset command
(0x88h) to the sensor after every power up.
4 History
Version
Date
1.0
Jan 17, 2014
by
Christian
Berg
Contents
initial version
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
Page 9 of 14
Created 1/17/2014 5:11:00 PM
Research & Development
5 Appendix
5.1 Grounding Concepts
Situation
•
•
•
•
high impedance (> 100kOhm) between sensor electronics and – housing
very sensitive measurement frontend (resolution is
fF)
shielding against noise necessary
the metallic case of the sensor must be electrically
connected to the power supply reference terminal
(GND) of the driving electronics
Concept #1
•
•
•
“indirect” connection via earth lines
easy to implement
keep impedance of earth connections as low as possible
Concept #2
•
•
•
direct connection which may optionally be connected
to earth
more laborious to implement
„direct“ connection better shielding possible
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
5.2 SPI Timing
5.2.1 Introduction
This chapter describes the actual time behavior of the new SPOT CDS5xxD sensor. The purpose of
this paper is to document the timing of the sensor.
5.2.2 Timing
5.2.2.1 Overview
Total measurement cycle
single measurement cycle
readout time
window
Figure 2: Timing Overview
Figure 2 shows the timing overview.
• Ch 1 (yellow) depicts the signal directly at the sensor
• Ch 2 (blue) equals the RDY (Ready) signal
The SPOT CDS5xxD sensor is configured to run “free” meaning that it
1. starts a measurement cycle consisting of 16 single measurements
2. computes the average of this 16 measurements
3. activates the RDY-signal
4. waits until the total measurement cycle time has elapsed
5. jumps back to step 1.
The corresponding timing is explained in more details in the following chapters.
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
5.2.3 Total Measurement Cycle
Figure 3: Measurement Cycle
•
•
Ch 1 (yellow) depicts the signal directly at the sensor
Ch 2 (blue) equals the RDY (Ready) signal
Figure 3 shows a measurement cycle. A measurement cycle is defined as the time between two consecutive negative edges of the Ready-signal. As can be seen from Figure 3, that time equals 682µs.
The timing source for the measurement cycle is an on chip oscillator. Although it is very stable it
shows a relatively large absolute frequency tolerance of about +/- 15% from chip to chip. Therefore the
cycle time can also vary in the range of about +/-15% from sensor to sensor.
5.2.4 Ready-signal
The Ready-signal is active low and it is set after the first measurement value is available. The Readysignal is reset by assigning the SS/-signal active. (SPI Slave Select) that means it is automatically
reset by a consecutive read of the measurement value.
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
5.2.5 Single Measurement Cycle
Figure 4: Single measurement cycle
•
•
Ch 1 (yellow) depicts the signal directly at the sensor
Ch 2 (blue) equals the RDY (Ready) signal
As can be seen from Figure 4, a single measurement cycle needs about 300µs. Again, this figure can
vary from sensor to sensor with up to +/-15%. During a single measurement cycle, the electronics
takes 16 single measurements and computes the average of them. Then it signals “new measurement
available” by asserting the Ready-signal low.
5.2.6 Readout Time Window
The “Readout Time Window”-Time is the time span between the negative going edge of the Readysignal and the beginning of a new measurement.
With
• “Readout Time Window”-time
= T_rd_out
• “Total Measurement Cycle”-time
= T_tot_cycle
• “Single Measurement Cycle“-time
= T_sgl_cycle
we get
T_rd_out = T_tot_cycle – T_sgl_cycle
For the example above : T_rd_out = 682µs - 300µs = 382µs.
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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Research & Development
5.2.7 Conclusion
The actual sensor configuration results in the following timing parameters:
• “Total Measurement Cycle”-time (T_tot_cycle)
680µs (+/-15%)
• “Single Measurement Cycle“-time (T_sgl_cycle)
300µs (+/-15%)
• “Readout Time Window”-time (T_rd_out)
380µs (+/-15%)
5.2.8 FAQ’s, Comments
Excerpt from an email:
We need to RDY signal example sequence. According to RDY signal shown in the attachment, measuring time is not 500 micro second, some time 200 micro and so on.
Could you give us additional information about RDY.
(See attached file: RDY timing analysis.ppt) below
• RDY signal is uneven timing
Figure 5: Excerpt from “RDY timing analysis.ppt”
As explained in the chapters above, the overall cycle time is the time between two consecutive negative going edges of the RDY signal. In the example above, this time equals 710µs and is very stable.
th
But there is one exception: The 4 low going edge of the RDY signal is missing because the readout
of the actual value was too late, or in other words, the next measurement cycle has started before the
Host was able to read the actual value.
The reason for the observed timing behavior of the RDY signal is the ignoring of the timing specifications.
SPOT CDS500D & CDS550D OEM Sensor SPI-Interface Specification_V1.0
Christian Berg, T based on Felix Mullis, TL
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