National Instruments 653X User Manual page 80

Using the Leading-Edge Protocol
ACK
REQ
Reference
Point
Initial State ACK is deasserted. The 653X device waits for an active REQ to indicate that the
peripheral device is ready. The peripheral device may optionally drive the first
data at this time. The transfer cannot begin until the peripheral asserts REQ: the
peripheral may either pulse REQ, or hold REQ high until the first ACK occurs.
If the peripheral pulses REQ, make sure to start the transfer on the 653X device
before the pulse occurs, to avoid missing the pulse.
1 The 653X device sends an ACK pulse when it is ready to receive data. The ACK
pulse width is fixed, assuming the peripheral device has deasserted the REQ
signal. Otherwise the ACK signal remains asserted until the REQ signal
deasserts.
2 After receiving at least the leading edge of the ACK pulse, the peripheral device
can strobe data into the 653X device by asserting REQ.
3 To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
4 The 653X device sends another ACK when it is ready for another input.
© National Instruments Corporation
1
2
Initial State
ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.
Action Steps
Figure 3-26. Leading Edge Input Handshaking Sequence
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Chapter 3
4
3
Timing Diagrams
653X User Manual