National Instruments 653X - Manual (Page 83)

Chapter 3 Timing Diagrams
653X User Manual 3-32 ni.com
Note With REQ edge latching enabled (default), the REQ edge determines when data will
be latched. Input data valid has to be held before an active going REQ edge a minimum of
t
rdi
ns. With REQ edge disabled, it has to be held t
adi
after the next active-going ACK signal
edge occurs.
Figure 3-29. Leading Edge Output Handshaking Sequence
Reference
Point Action Steps
Initial State ACK is deasserted.
1 The 653X device sends the ACK pulse after driving output data to indicate that it
has new, valid output data. The ACK pulse width is fixed, assuming the peripheral
device has deasserted the REQ signal. Otherwise, the ACK signal remains
asserted until the peripheral device deasserts the REQ signal.
2 Once the data is latched, the peripheral device must respond with an active-going
REQ signal edge to request additional data.
3 To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
ACK
REQ
1
3
2
ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.
Initial State