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AD9545

Data Sheet

Rev. A | Page 118 of 157

relaxes this requirement. The ping pong TDC processor

monitors the two auxiliary TDCs and performs the requisite

control functions necessary to observe two consecutive rising

edge events without requiring advance knowledge of the input

period. Because the ping pong TDC processor requires time to

execute the control functions, the user must ensure at least 10 µs

between a pair of successive rising edges. Otherwise, the ping

pong TDC may miss or incorrectly interpret an edge event.

The ping pong TDC is especially useful for monitoring the

occurrence of an input signal with arbitrarily spaced pulses. For

example, suppose an input signal is a pulse that identifies some

random asynchronous event with a long wait time. Because the

ping pong TDC routes to the user time stamp processor, the

user can set up a pulse monitor with an extended span enabling

the capture of a random event.

This extended span is possible because user time stamps have a

40-bit integer part for which an increment of one corresponds

to the period of its time base. Furthermore, the time base

period is established by one of the auxiliary NCOs. As such, the

user may, for example, program the auxiliary NCO for 1 Hz,

yielding a user time stamp span of 240 sec (~35,000 years) with

1 ps granularity. The ability to use a 1 Hz time base enables the

ping pong TDC to monitor input pulses having an extended

span between pulses.

Mx PIN

SWITCH

MATRIX

AUX

TDC 0

TIME

STAMPS

PING PONG

TDC

PROCESSOR

TIME

STAMPS

AUX

TDC 1

TO DIGITAL

CROSS POINT

MUX

TIME

STAMPS

CLOCK SOURCE

(DERIVED FROM SYSTEM CLOCK)

PING PONG TDC

Figure 92. Ping Pong TDC Block Diagram

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