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5.10.8 Capture Timers (IOW24  V1.0.3.0 only)

IO-Warrior24 has two capture timers that allow the

measurement of the time between rising and falling

edges of a signal. The resolution of the timers is

4sec, the accuracy depends on the USB clock

from which IOW24 derives its internal clock with

a tolerance of ±1.5%.

The capture timers use a 24 bit free running

counter that gets incremented every 4

microseconds. When a rising or falling edge occurs

on a timer input (i.e. the logic level on the pin goes

from low to high or from high to low) the value of

the counter is stored and send in a report. The

relative timing between events can be measured.

When reaching maximum count the counter rolls

over and starts again from zero. To measure events

longer apart than 67.108864s additional time

keeping must be done on the host computer.

Two capture timers are available that can be used

individually or at the same time. They use the same

24 bit counter, so no timing offset between the two

timers exists.

Periodic events up to 250Hz on one timer or up to

125Hz on both timers simultaneously can be

tracked with no edges lost, if no other special mode

functions of IOW24 are used at the same time. At

higher frequencies events can be lost due to

overrun (see below).

The maximum frequency for periodic signals that

can be handled without IOW24 locking up due to

internal overload is above 100kHz (lockup is not

permanent when the signal frequency drops

IOW24 will resume to work normally). To reliably

measure periodic signals faster than a couple

100Hz it is recommended to use an external gating

circuit.

Using the LED matrix function or sending/

receiving data via IIC during a capture timer

measurement can cause the timer value to be

corrupted due to too high internal processing load.

The capture timers are enabled and disabled by a

report with ID $28 that is sent to interface 1:

enable = $00 disables both timers

enable bit 0 = 1 enables capture timer A

enable bit 1 = 1 enables capture timer B

Always disable the timers before switching to a

different enable state, i.e. if only timer A is in use

and you want to activate timer B too, first disable

the timers (enable = $00) before activating both

timers (enable = $03). This applies for all

combinations of switching between enable

combinations of the timers.

The capture timers can not be used at the same

time as the RC5 receiver. Timer B shares the pin

with the IIC function and can not be used at the

same time as the IIC function.

Timer data is returned when a rising and/or falling

edge was detetced on either timer input pin. Timer

A uses report ID $29 to return its data, timer B uses

report ID $2A:

flags is a bitfield that signals which edges have

been detected and if there was an overrun

condition. An overrun occurs if another rising or

falling edge is detected on the same timer before

the data for the last edge data could be send via

USB. IO-Warrior24 can return one data packet

every 8msec, this limit applies to the data

combined from both capture timers and all other

special mode functions of IOW24.

In case of an overrun the first edge that has been

detected is reported, subsequent edges are ignored

until the data has been send off via USB. There is

no guarantee that IOW24 can reliably detect an

overrun if the events happen too fast.

The meaning of the flag bits is as follows:

7 - Capture B rising edge overrun

6 - Capture B falling edge overrun

5 - Capture A rising edge overrun

4 - Capture A falling edge overrun

3 - Capture B rising edge detected

2 - Capture B falling edge detected

1 - Capture A rising edge detected

0 - Capture A falling edge detected

The report contains data fields for a fall and rise

event of the respective timer. The flags indicate if

the data is valid, i.e. if the flag for rising edge

detected is not set then the rising edge data is not

valid.

The flag bits for both timers are always included in

both reports.

fall0, fall1, fall2, rise0, rise1, rise2 contain the

counter status at the time of the event, xx0 is the

least significant byte, xx2 is the most significant

byte.

ReportID

$28 out

enable $00

$00 $00

$00

ReportID

$29 in

flags fall0

fall1 fall2

rise0 rise1

rise2

ReportID

$2A in

flags fall0

fall1 fall2

rise0 rise1

rise2

V 1.1.0, December 2nd 2013, for chip revision V1.0.3.0 and up