ST92195D Datasheet, PDF (179/249 Pages) STMicroelectronics – 48-96 Kbyte ROM HCMOS MCU WITH ON-SCREEN DISPLAY AND TELETEXT DATA SLICER

ST92195C/D - SERIAL PERIPHERAL INTERFACE (SPI)
SERIAL PERIPHERAL INTERFACE (Cont’d)
The data on the SDA line is sampled on the low to
high transition of the SCL line.
SPI working with an I²C-bus
To use the SPI with the I²C-bus protocol, the SCK
line is used as SCL; the SDI and SDO lines, exter-
nally wire-ORed, are used as SDA. All output pins
must be configured as open drain (see Figure 95).
Table 32. illustrates the typical I²C-bus sequence,
comprising 5 phases: Initialization, Start, Trans-
mission, Acknowledge and Stop. It should be not-
ed that only the first 8 bits are handled by the SPI
peripheral; the ACKNOWLEDGE bit must be man-
aged by software, by polling or forcing the SCL
and SDO lines via the corresponding I/O port bits.
During the transmission phase, the following I²C-
bus features are also supported by hardware.
Clock Synchronization
In a multimaster I²C-bus system, when several
masters generate their own clock, synchronization
is required. The first master which releases the
SCL line stops internal counting, restarting only
when the SCL line goes high (released by all the
other masters). In this manner, devices using dif-
Figure 98. SPI Arbitration
ferent clock sources and different frequencies can
be interfaced.
Arbitration Lost
When several masters are sending data on the
SDA line, the following takes place: if the transmit-
ter sends a “1” and the SDA line is forced low by
another device, the ARB flag (SPICR.5) is set and
the SDO buffer is disabled (ARB is reset and the
SDO buffer is enabled when SPIDR is written to
again). When BMS is set, the peripheral clock is
supplied through the INT2 line by the external
clock line (SCL). Due to potential noise spikes
(which must last longer than one INTCLK period to
be detected), RX or TX may gain a clock pulse.
Referring to Figure 97, if device ST9-1 detects a
noise spike and therefore gains a clock pulse, it
will stop its transmission early and hold the clock
line low, causing device ST9-2 to freeze on the 7th
bit. To exit and recover from this condition, the
BMS bit must be reset; this will cause the SPI logic
to be reset, thus aborting the current transmission.
An End of Transmission interrupt is generated fol-
lowing this reset sequence.
ST9-1
INTERNAL SERIAL
CLOCK
ST9-2
INTERNAL SERIAL
CLOCK
0
MSPI
CONTROL
LOGIC
1
BHS
SCK
INT 2
SCK
INT 2
0
MSPI
CONTROL
LOGIC
1
BHS
ST9-2-SCK
1
2
3
4
5
6
7
SPIKE
ST9-1-SCK
1
2
3
4
5
6
7
8
VR001410
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