SER_IRQ Cycle Control
There are two modes of operation for the SER_IRQ Start Frame.
1)
Quiet (Active) Mode: Any device may initiate a Start Frame by driving the SER_IRQ low for one clock, while
the SER_IRQ is Idle. After driving low for one clock the SER_IRQ must immediately be tri-stated without at
any time driving high. A Start Frame may not be initiated while the SER_IRQ is Active. The SER_IRQ is Idle
between Stop and Start Frames. The SER_IRQ is Active between Start and Stop Frames. This mode of
operation allows the SER_IRQ to be Idle when there are no IRQ/Data transitions which should be most of the
time.
Once a Start Frame has been initiated the Host Controller will take over driving the SER_IRQ low in the next
clock and will continue driving the SER_IRQ low for a programmable period of three to seven clocks. This
makes a total low pulse width of four to eight clocks. Finally, the Host Controller will drive the SER_IRQ back
high for one clock, then tri-state. Any SER_IRQ Device (i.e., The LPC47M14x) which detects any transition
on an IRQ/Data line for which it is responsible must initiate a Start Frame in order to update the Host
Controller unless the SER_IRQ is already in an SER_IRQ Cycle and the IRQ/Data transition can be delivered
in that SER_IRQ Cycle.
2)
Continuous (Idle) Mode: Only the Host controller can initiate a Start Frame to update IRQ/Data line
information. All other SER_IRQ agents become passive and may not initiate a Start Frame. SER_IRQ will be
driven low for four to eight clocks by Host Controller. This mode has two functions. It can be used to stop or
idle the SER_IRQ or the Host Controller can operate SER_IRQ in a continuous mode by initiating a Start
Frame at the end of every Stop Frame.
An SER_IRQ mode transition can only occur during the Stop Frame. Upon reset, SER_IRQ bus is defaulted to
Continuous mode, therefore only the Host controller can initiate the first Start Frame. Slaves must
continuously sample the Stop Frames pulse width to determine the next SER_IRQ Cycle’s mode.
SER_IRQ Data Frame
Once a Start Frame has been initiated, the LPC47M14x will watch for the rising edge of the Start Pulse and start
counting IRQ/Data Frames from there. Each IRQ/Data Frame is three clocks: Sample phase, Recovery phase, and
Turn-around phase. During the Sample phase the LPC47M14x must drive the SER_IRQ low, if and only if, its last
detected IRQ/Data value was low. If its detected IRQ/Data value is high, SER_IRQ must be left tri-stated. During the
Recovery phase the LPC47M14x must drive the SER_IRQ high, if and only if, it had driven the SER_IRQ low during
the previous Sample Phase. During the Turn-around Phase the LPC47M14x must tri-state the SER_IRQ. The
LPC47M14x will drive the SER_IRQ line low at the appropriate sample point if its associated IRQ/Data line is low,
regardless of which device initiated the Start Frame.
The Sample Phase for each IRQ/Data follows the low to high transition of the Start Frame pulse by a number of
clocks equal to the IRQ/Data Frame times three, minus one. (e.g. The IRQ5 Sample clock is the sixth IRQ/Data
Frame, (6 x 3) - 1 = 17th clock after the rising edge of the Start Pulse).
SER_IRQ Sampling Periods
SER_IRQ PERIOD
SIGNAL SAMPLED
Not Used
IRQ1
# OF CLOCKS PAST START
1
2
2
5
3
4
5
6
7
8
9
10
11
12
13
14
15
16
nIO_SMI/IRQ2
IRQ3
8
11
14
17
20
23
26
29
32
35
38
41
44
47
IRQ4
IRQ5
IRQ6
IRQ7
IRQ8
IRQ9
IRQ10
IRQ11
IRQ12
IRQ13
IRQ14
IRQ15
SMSC DS – LPC47M14X
Page 99
Rev. 03/19/2001
SMSC [ SMSC CORPORATION ]
