SCA3000 Series
followed by one zero bit. The later 8 bits in the MOSI line contain data for a write operation and are
‘don’t-care’ for a read operation. Bits from MOSI line are sampled in on the rising edge of SCK and
bits to MISO line are latched out on falling edge of SCK.
The first bits in the MISO line are the frame error bit (SPI_FRAME, bit 2) of the previous SPI frame
and odd parity bit (PAR, bit 8). Parity is calculated from data which is currently sent. Bit 7 is always
‘1’. The later 8 bits contain data for a read operation. During the write operation, these data bits are
previous data bits of the addressed register.
For write commands, data is written into the addressed register on the rising edge of CSB. If the
command frame is invalid as described in the section data will not be written into the register
(please see "error conditioning" in section 4.1.2).
For read commands, data is latched into the internal SPI output register (shift register) on the 8th
rising edge of SCK. The output register is shifted out MSB first over MISO output.
When the CSB is high state between data transfers, the MISO line is in the high-impedance state.
4.1.2 SPI bus error conditioning
While sending an SPI frame, if the CSB is raised to 1
-
-
before sending 16 SCKs or
the number of SCK pulses is not divisible by 8,
the frame error is activated and the frame is considered invalid. The status bit
STATUS.SPI_FRAME is set to indicate the frame error condition. During the next SPI, the frame
error bit is sent out as SPI_FRAME bit (see SPI_FRAME in MISO line in Figure 6).
STATUS.SPI_FRAME bit is reset, if correct frame is received.
When an invalid frame is received, the last command is simply ignored and the register contents
are left unchanged. If frame error happens while sending multiple samples in ring buffer mode, only
the last output value is considered invalid.
4.1.3 Examples of SPI communication
4.1.3.1 Example of register read
An example of 11 bit X-axis acceleration read command is presented in Figure 7. The master gives
the register address to be read via the MOSI line: '05' in hex format and '000101' in binary format,
register name is X_MSB (X-axis MSB frame). 7th bit is set to '0' to indicate the read operation.
The sensor replies to a requested operation by transferring the register content via MISO line. After
transferring the asked X_MSB register content, the master gives next register address to be read:
'04' in hex format and '000100' in binary format, register name is X_LSB (X-axis LSB frame). The
sensor replies to the requested operation by transferring the register content MSB first.
Figure 7. An example of SPI read communication.
VTI Technologies Oy
www.vti.fi
PRELIMINARY - Subject to changes
Doc.Nr. 8257300A.06
27/ 45
Rev.A.06
VTI [ VTI TECHNOLOGIES ]
