Technical Note 15 1(5)
SPI Interface Specification
OBJECTIVE
This document specifies the Serial Peripheral Interface (SPI) that is used in the SCA61T, SCA100T,
SCA103T, SCA1000, and SCA1020 –series sensors.
THE SPI INTERFACE
A Serial Peripheral Interface (SPI) system consists of one master device and one or more slave devices. The
master is defined as a microcontroller providing the SPI clock and the slave as any integrated circuit receiving
the SPI clock from the master. The ASIC in VTI Technologies’ products always operates as a slave device in
master-slave operation mode.
The SPI has a 4-wire synchronous serial interface. Data communication is enabled with a low active Slave
Select or Chip Select wire (CSB). Data is transmitted with a 3-wire interface consisting of wires for serial data
input (MOSI), serial data output (MISO) and serial clock (SCK).
DATA OUT (MOSI)
DATA IN (MISO)
SERIAL CLOCK (SCK)
SS0
SS1
SS2
SS3
MASTER
MICROCONTROLLER
SI
SO
SCK
CS
SLAVE
SI
SO
SCK
CS
SI
SO
SCK
CS
SI
SO
SCK
CS
Figure 1. Typical SPI connection
The SPI interface in VTI products is designed to support any microcontroller that uses SPI bus.
Communication can be carried out by software or hardware based SPI. Please note that in the case of
hardware based SPI, the received acceleration data is 11 bits. The SPI interface is used for testing and
calibration purposes, and it can also be used in the final application. Some of the test and calibration
commands are disabled in normal use, and are not documented here. The data transfer uses the following 4-
wire interface:
MOSI master out slave in µP → ASIC
MISO master in slave out ASIC → µP
SCK serial clock µP → ASIC
CSB chip select (low active) µP → ASIC
1
9 Sep 2005 VTI Technologies reserves all rights to modify this document without prior notice.
Technical Note 15 2(5)
Each transmission starts with a falling edge of CSB and ends with the
rising edge. During transmission, commands and data are controlled by
SCK and CSB according to the following rules:
• commands and data are shifted; MSB first, LSB last
• each output data/status bits are shifted out on the falling edge of SCK (MISO line)
• each bit is sampled on the rising edge of SCK (MOSI line)
• after the device is selected with the falling edge of CSB, an 8-bit command is received. The command
defines the operations to be performed
• the rising edge of CSB ends all data transfer and resets internal counter and command register
• if an invalid command is received, no data is shifted into the chip and the MISO remains in high
impedance state until the falling edge of CSB. This reinitializes the serial communication.
• In order to perform other commands than those listed in Table 1, the lock register content must be set
correctly. If such a command is fed without setting the correct lock register content, no data will be
shifted into the chip and the MISO remains in high impedance state until the falling edge of CSB.
• data transfer to MOSI continues immediately after receiving the command in all cases where data is to
be written to ASIC’s internal registers
• data transfer out from MISO starts with the falling edge of SCK immediately after the last bit of the SPI
command is sampled in on the rising edge of SCK
• maximum SPI clock frequency is 500kHz
• maximum data transfer speed for RDAX and RDAY is 6600 samples per sec / channel
SPI command can be either an individual command or a combination of command and data. In the case of
combined command and data, the input data follows uninterruptedly the SPI command and the output data is
shifted out parallel with the input data.
7 6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
CSB
SCK
M ISO
M OSI
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H IGH IM PED ANCE
COM M AND DATA IN
DATA OU T
Figure 2. Command and data transmission over the SPI
After power up, the circuit starts to operate in Measure mode. This is the normal operation mode that is used
for the applications.
19 Sep 2005 VTI Technologies reserves all rights to modify this document prior notice.
Technica 3(5)
DIGITAL INTERFACE SPECIFICATION (TABLE 1)
Parameter Condition Min Typ Max Units
Digital output load @ 500 kHz 1 nF
SPI clock frequency (1 500 kHz
Internal AD conversion 150 µs
Data transfer time @ 500 kHz 38 µs
Note1. Minimum SPI frequency depends on the master
microcontroller clock frequency
THE SPI COMMANDS
The SPI interface uses an 8-bit instruction (or command) register.
The list of commands is given in Table 2.
Table 2. SPI commands. The commands in Italic are
in use in the 2-axis SCA100T only.
Command Command format Description:
MEAS 00000000 Measure mode (normal operation mode after power on)
RWTR 00001000 Read and write temperature data register
STX 00001110 Activate Self test for X-channel
STY 00001111 Activate Self test for Y-channel
RDAX 00010000 Read X-channel acceleration through SPI
RDAY 00010001 Read Y-channel acceleration through SPI
Measure mode (MEAS) is standard operation mode after power-up. During normal operation, MEAS
command is the exit command from Self test.
Read temperature data register (RWTR) reads temperature data register during normal operation without
effecting the operation. Temperature data register is updated every 150 µs. The load operation is disabled
whenever the CSB signal is low, hence CSB must stay high at least 150 µs prior the RWTR command in order
to guarantee correct data. The data transfer is presented in Figure 3, and the data is transferred MSB first. In
normal operation, it does not matter what data is written into temperature data register during the RWTR
command and hence writing all zeros is recommended.
Self test for X-channel (STX) activates the self test function for the X-channel (Channel 1). The Internal
charge pump is activated and a high voltage is applied to the X-channel acceleration sensor element
electrode. This causes the electrostatic force that deflects the beam of the sensing element and simulates the
acceleration to the positive direction. The X-channel self-test is de-activated by giving the MEAS command.
Self test for Y-channel (STY) activates the self test function for the Y-channel (Channel 2). The internal
charge pump is activated and a high voltage is applied to the Y-channel acceleration sensor element
electrode. This causes the electrostatic force that deflects the beam of the sensing element and simulates the
acceleration to the positive direction. The Y-channel self-test is de-activated by giving the MEAS command.
Note! This command is valid for the 2-axis SCA100T only.
Read X-channel acceleration (RDAX) accesses the AD converted X-channel (Channel 1) acceleration signal
stored in acceleration data register X. During normal operation, acceleration data register X is reloaded every
150 µs. The load operation is disabled whenever the CSB signal is low, hence CSB must stay high at least
150 µs prior the RDAX command in order to guarantee correct data. Data output is an 11-bit digital word that
is fed out MSB first and LSB last. (see Figures 3 and 4).
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l Note 15
Technical Note 15 4(5)
Read Y-channel acceleration (RDAY) accesses the AD converted Y-channel
(Channel 2) acceleration signal stored in acceleration data register Y. During
normal operation acceleration data register Y is reloaded every 150 µs. The
load operation is disabled whenever the CSB signal is low, hence CSB must
stay high at least 150 µs prior the RDAY command in order to guarantee
correct data. Data output is an 11-bit digital word that is fed out MSB first and
LSB last. Note! This command is valid for the 2-axis SCA100T only.
RDAX COMMAND AND DATA TRANSMISSION
OVER THE SPI (FIGURE 3)
MSB
MISO in high-impedance
during 8-bit command.
MOSI data = 16 (in decimal format)
MISO data = 975 (in decimal format)
LSB
0 1 1 1 1 0 0 1 1 1 1
0 0 0 1 0 0 0 0
SPI BUS TIMING DIAGRAM (FIGURE 4)
CSB
SCK
MOSI
MISO
TLS1 TCH
THOL TSET
TVAL1 TVAL2 TLZ
TLS2 TLH
MSB in
MSB out
LSB in
LSB outDATA out
DATA in
TCL
19 Sep 2005 VTI Technologies reserves all rights to modify this document without prior notice.
Technical Note 15 5(5)
DC CHARACTERISTICS OF THE SPI INTERFACE (TABLE 3)
Parameter Conditions Symbol Min Typ Max Unit
Input terminal CSB
Pull up current VIN = 0 V IPU 13 22 35 µA
Input high voltage VIH 4 Vdd+0.3 V
Input low voltage VIL -0.3 1 V
Hysteresis VHYST 0.23*Vdd V
Input capacitance CIN 2 pF
Input terminal MOSI, SCK
Pull down current VIN = 5 V IPD 9 17 29 µA
Input high voltage VIH 4 Vdd+0.3 V
Input low voltage VIL -0.3 1 V
Hysteresis VHYST 0.23*Vdd V
Input capacitance CIN 2 pF
Output terminal MISO
Output high voltage I > -1mA VOH Vdd-0.5 V
Output low voltage I < 1 mA VOL 0.5 V
Tristate leakage 0 < VMISO < Vdd ILEAK 5 100 pA
Supply voltage is 5 V unless otherwise noted. Current flowing into the circuit have positive values.
AC CHARACTERISTICS OF THE SPI INTERFACE (TABLE 4)
Parameter Conditions Symbol Min Typ Max Unit
Terminal CSB, SCK
Time from CSB (10%) to SCK (90%)(1 TLS1 120 ns
Time from SCK (10%) to CSB (90%)(1 TLS2 120 ns
Terminal SCK
SCK low time Load capacitance
at MISO < 2 nF
TCL 1 µs
SCK high time Load capacitance
at MISO < 2 nF
TCH 1 µs
Terminal MOSI, SCK
Time from changing MOSI (10%,
90%) to SCK (90%)(1. Data setup time
TSET 30 ns
Time from SCK (90%) to changing
MOSI (10%,90%)(1. Data hold time
THOL 30 ns
Terminal MISO, CSB
Time from CSB (10%) to stable MISO
(10%, 90%)(1.
Load capacitance
at MISO < 15 pF
TVAL1 10 100 ns
Time from CSB (90%) to high
impedance state of MISO(1.
Load capacitance
at MISO < 15 pF
TLZ 10 100 ns
Terminal MISO, SCK
Time from SCK (10%) to stable MISO
(10%, 90%)(1.
Load capacitance
at MISO < 15 pF
TVAL2 100 ns
Terminal CSB
Time between SPI cycles, CSB at
high level (90%)
TLH 15 µs
When using SPI commands RDAX,
RDAY, RWTR: Time between SPI
cycles, CSB at high level (90%)
TLH 150 µs
(1 not production tested
19 Sep 2005 VTI Technologies reserves all rights to modify this document without prior notice.
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150
(s
Input terminal CSB
Input terminal MOSI, SCK
Output terminal MISO
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