
SMSC DS – LPC47M14X 
Page 99 
Rev. 03/19/2001 
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 
1 
Not Used 
2 
IRQ1 
3 
nIO_SMI/IRQ2 
4 
IRQ3 
5 
IRQ4 
6 
IRQ5 
7 
IRQ6 
8 
IRQ7 
9 
IRQ8 
10 
IRQ9 
11 
IRQ10 
12 
IRQ11 
13 
IRQ12 
14 
IRQ13 
15 
IRQ14 
16 
IRQ15 
# OF CLOCKS PAST START 
2 
5 
8 
11 
14 
17 
20 
23 
26 
29 
32 
35 
38 
41 
44 
47