33661 Datasheet, PDF (14/21 Pages) Freescale Semiconductor, Inc – Local Area Network (LIN) Enhanced Physical Interface with Selectable Slew Rate

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
As described below and depicted in Figure 15 and Table 5
on 15, the 33661 has two operational modes, Normal and
Sleep. Normal mode may be adjusted to improve radiated
emissions by changing the slew rate of the LIN bus output to
Fast or Slow mode. In addition, there are two transitional
modes: Awake Mode, which allows the device to go in
Normal or Slow mode, and Wait Slow mode, which is a
temporary state before the device enters the Slow mode.
NORMAL MODE
In the Normal mode, the 33661 has slew rate and timing
compatible with the LIN protocol specification and operates
from 1.0 kbps to 20 kbps. This mode is selected after Sleep
mode by setting the TXD pin HIGH prior to setting EN from
LOW to HIGH. Once Normal mode is selected, it is
impossible to select the Slow mode unless the 33661 is set
to Sleep mode.
Slow Mode
In the Slow mode, the slew rate is around half the normal
slew rate, and bus speed operation ranges from 1.0 kbps to
10 kbps. The radiated emission is significantly reduced
compared to the already excellent emission level of the
Normal mode. Slow mode is entered after Sleep mode by
setting the TXD pin LOW prior to setting EN from LOW to
HIGH. Once the Slow mode is selected, it is impossible to
select the Normal mode unless the device is set to Sleep
mode.
Fast Mode
In the Fast mode, the slew rate is around 10 times faster
than the Normal mode. This allows very fast data
transmission (> 100 kbps) — for instance, for electronic
control unit (ECU) tests and microcontroller program
download. The bus pullup resistor might be reduced to
ensure a correct RC time constant in line with the high baud
rate used.
Fast mode can be selected from either Normal or Slow
mode. Fast mode is entered via a special sequence (called
toggle function) as follows: TXD and EN pins set LOW, then
TXD pulled HIGH, and at the EN pin LOW-to-HIGH transition,
the device enters into the Fast Baud Rate. The duration of
this sequence must be less than 35 µs. The toggle function is
described in Figure 13, 11. Once in the Fast mode, two
different procedures will bring the device back to the
previously selected mode (Normal or Slow):
• The toggle function already described.
• A glitch on EN where t₂ < 5.0 µs also resets the device to
the previously selected mode (Normal or Slow)
(Figure 13).
SLEEP MODE
In the Sleep mode, the transmission path is disabled and
the 33661 is in low power mode. Supply current from V_SUP is
very low. Wake-up can occur from LIN bus activity from node
internal wake-up through the EN pin and from the WAKE
input pin.
In the Sleep mode, the 33661 has an internal 20 µA pullup
source to VSUP. This avoids the high current path from the
battery to ground in the event the bus is shorted to ground.
(Refer to succeeding paragraphs describing wake-up
behavior.)
DEVICE POWER-UP (AWAKE TRANSITIONAL
MODE)
At power-up (V_SUP rises from zero), the 33661
automatically switches to the Awake transitional mode. It
switches the INH pin to HIGH state and RXD to LOW state.
The MCU of the application will then confirm Normal or Slow
mode by setting the TXD and EN pins appropriately.
DEVICE WAKE-UP EVENTS
The 33661 can be awakened from Sleep mode by three
wake-up events:
• Remote wake-up via LIN bus activity
• Internal node wake-up via the EN pin
• Toggling the WAKE pin
Remote Wake from LIN Bus (Awake Transitional Mode)
The LIN bus wake-up is recognized by a recessive-to-
dominant transition, followed by a dominant level with a
duration greater than 70 µs, followed by a dominant-to-
recessive transition. This is illustrated in Figures 11 and 12 on
11. Once the wake-up is detected, the 33661 enters the
Awake transitional mode, with INH HIGH and RXD pulled
LOW.
Wake-Up from Internal Node Activity (Normal or Wait
Slow Mode)
The 33661 can wake up by internal node activity through
a LOW-to-HIGH transition of the EN pin. When EN is
switched from LOW to HIGH, the device is awakened and
enters either the Normal or the Wait Slow transitional mode
depending on the level of TXD input. The MCU must set the
TXD pin LOW or HIGH prior to waking up the device through
the EN pin.
Wake-Up from WAKE Pin (Awake Transitional Mode)
If the WAKE input pin is toggled, the 33661 enters the
Awake transitional mode, with INH HIGH and RXD pulled
LOW.
33661
14
Analog Integrated Circuit Device Data
Freescale Semiconductor