MAX9257/MAX9258
UART-to-I
2
C Converter
The UART-to-I
2
C converter accepts UART read or write
packets issued by the ECU and converts them to an I
2
C
master protocol when in base mode. A slave can use an
ACK or NACK to indicate a busy or wait state, but cannot
hold SCL low to indicate a wait state. Multiple slaves are
supported. The UART-to-I
2
C conversion delay is less
than 22 UART bit times and needs to be taken into
account when setting the ETO and STO timeout periods
for read commands. UART-to-I
2
C converter converts
standard UART format to standard I
2
C format (Figure
25). This includes data-bit ordering conversion because
UART transmits the LSB in first while I
2
C transmits the
MSB first. UART/I
2
C read delay is a maximum 34 bit
times when reading from an I
2
C peripheral.
The MAX9257/MAX9258 store their own 7-bit device
addresses in register REG5. All packets not addressed
to the MAX9257/MAX9258 are forwarded to the UART-
to-I
2
C converter. The I
2
C interfaces (SDA and SCL) are
open drain and actively drive a low state. When idle,
SDA and SCL are high impedance and pulled high by a
pullup resistor. SDA and SCL are idle when packets are
addressed to the MAX9257 or MAX9258. SDA and SCL
are also idle when the I
2
C interface is programmed to
be disabled.
Bypass Mode (Details)
In bypass mode, ECU activity and UART communica-
tion from the camera reset the ETO and CTO timers.
This allows the control channel to stay in bypass as
long as there is camera activity. In base mode, only
ECU activity resets the ETO and CTO timers.
Bypass mode temporarily or permanently blocks pro-
gramming of the MAX9257/MAX9258. Bypass mode
allows only UART programming of peripheral device by
ECU. There is no I
2
C connection in bypass mode. Bypass
mode is entered by writing a 0 to INTMODE and by writ-
ing a 1 to INTEN (Table 23). Bypass mode disables ECU
programming of the MAX9257/MAX9258 to allow any
UART communication protocol with the peripheral device.
Once bypass mode is entered, the MAX9257/MAX9258
stay in bypass mode until CTO times out.
In bypass mode, the STO and ETO timers determine
the control channel duration. CTO timer determines
whether to revert back to base mode or not, and EF is
not recognized.
A useful setting in bypass mode is to set STO > CTO >
ETO because this setting is an alternative to permanent
bypass (Figure 24). Use this setting to stay in bypass
mode to avoid the overhead of entering from base mode
every time the control channel opens. If the ECU uses the
channel within a CTO timeout, ETO is activated and then
ETO times out before CTO. The channel closes because
ETO times out, but channel stays in bypass mode
because CTO does not time out. At the next vertical
blanking time, bypass mode continues with CTO reset
and the ECU can immediately send commands to the
camera. If the ECU or camera does not use the channel,
CTO times out before STO. STO closes the channel
(because ETO is not enabled) if no communication is
34 ______________________________________________________________________________________
Fully Programmable Serializer/Deserializer
with UART/I
2
C Control Channel
