
4-8
Integration is enabled by the rising edge of the input control
signal INT/HOLD. Within 20
μ
s after the integrate input
reaches a logic high level, the output of the integrator will fall
to approximately V
RESET
, 0.125V. The output of the
integrator is an analog voltage.
Differential to Single-Ended Converter
This circuit takes the differential output of the integrators
(through the test-multiplexer circuit) and provides a signal
that is the sum of the two signals. This technique is used to
improve the noise immunity of the system.
Output Buffer
This output amplifier is the same amplifier circuits as the
input amplifier used to interface with the sensors. For
diagnostic purposes when the output of the antialias filter is
being evaluated, this amplifier is in the power down mode.
Test Multiplexer
This circuit receives the positive and negative outputs from
the integrator, together with the outputs from different parts
of the IC. The Test Mux output is controlled by the fifth
programming word of the communications protocol. This
multiplexes the switch capacitor filter output, the gain control
output and the antialias filter output.
SPI Communications Protocol
Communicating to the Knock Sensor via the SPI Bus
(MOSI). A chip select pin (CS) is used to enable the chip,
which, in conjunction with the SPI clock (SCK), which moves
an eight bit programming word. Five different programming
words are used to set the following internal programmable
registers: GAIN, BANDPASS FREQUENCY FILTER,
INTEGRATOR TIME CONSTANT, CHANNEL SELECT, SO
output mode, and TEST MODES.
When chip select (CS) goes low, on the next falling edge of
the SPI clock (SCK), data is latched into the SPI register.
The data is shifted with the most significant bit first and least
significant bit last. Each word is divided into two parts: first
the address and then the value. Depending on the function
being controlled, the address is 2 or 3 bits, and the value is
either 5 or 6 bits long. All five programming words can be
entered into the IC during the HOLD mode of operation. The
integration or hold mode of operation is controlled by the
INT/HOLD input signal.
Programming Words
1. BandPassFilterFrequency:Definesthecenterfrequency
of the Band Pass Filter in the system. The first 2 bits are
used for the address and the last 6 bits are used for its
value.00FFFFFFExample:00001010wouldbetheBand
Pass Filter at a center frequency of 1.78kHz (bit value of
10 in Table 3).
2. Gain Control: defines the value of the gain stage attenua-
tion of gain setting. The first 2 bits are again used for the
address and the last 6 bits for its value. 10GGGGGG Ex-
ample: 10010100 would be the Gain Control (10 for the
first two bits) with an attenuation of 0.739 (bit value of 20
in Table 3).
3. Integrator Time Constant: Defines the Integration Time
Constant for the system. The first 3 bits are used for the
address and the last 5 bits for the value. 110TTTTT Ex-
ample: 11000011 would be the Integrator Time Constant
(110 for the first 3 bits) and an Integration Time Constant
of 55
μ
s (bit value 3 in Table 3).
4. Test/ChannelSelectControl:Againthefirstthreebits,111
are the address for this function, and the last five bits de-
fine the functions that may be programmed. Example:
111B4B3B2B1B0; The options are:
A) If B0 is “0”, than channel 0 is selected. If B0 is “1”;
than channel 1 is selected as the input.
B) The remaining bits are used for selection of the various
diagnostic modes. TEST pin (14) = low. Not applicable
in Run Mode.
5. Prescaler/SOterminalstatus:Definesthedivisionratioof
the internal frequency prescaler and the status of the SO
terminal, pin 11. P1 to P4 bits define the frequency that
may be used with an external clock. The status of the
three state SO pin is set by the last, Z bit.
01P5P4P3P2P1Z; Example: 0100000, Note, in this case
bit P5 is not used. (01 for the first 2 bits sets the Prescal-
er/SO function) P1 to P4 set Prescaler for a clock
frequency of 4MHz and the last bit sets the S0 terminal to
an active state.
HIP9011