sc26c94 NXP Semiconductors, sc26c94 Datasheet - Page 33

• Manufacturer Part Number: sc26c94
• Description: Quad Universal Asynchronous Receiver/transmitter Quart
• Manufacturer: NXP Semiconductors

Philips Semiconductors
addressed register. The generation of DACKN begins with the start
of a bus cycle (Read, Write or Interrupt Acknowledge) and then
requires two edges of the X1 clock plus typically 70ns for its
assertion.
In this mode the writing of data to the QUART registers occurs on
the falling edge of DACKN or the rising edge of the combination of
CEN and WRN which ever occurs first. This requires that the data
to be written to the QUART registers be valid with respect to the
leading edge of the combination of CEN and WRN. (In the
synchronous mode it is the trailing edge)
IACKN updates the CIR (Current Interrupt Register) and places the
Interrupt Vector or Modified Interrupt Vector on the bus if the
Interrupt Vector is used.
The Synchronous Interface
In this mode the DACKN and IACKN are usually not used. Here
data is written to the QUART on the trailing edge of the
combination of CEN and WRN. The placing of data on the bus
during a read cycle begins with the leading edge of the combination
of CEN and RDN.
The read cycle will terminate with the rise of CEN or RDN which
ever one occurs first. In this mode bus cycles are usually setup to be
the minimum time required by the QUART and hence will be faster
than bus cycles that are defined by the DACKN signal. DACKN
should be turned off in this mode.
1995 May 1
Quad universal asynchronous receiver/transmitter (QUART)
33
When IACKN is not used or is not available the command at 2Ah
should be used to update the CIR (Current Interrupt Register). This
register is normally updated by IACKN in response to the IRQN.
Note that the CIR is not updated by IRQN since there could be a
long time between the assertion of IRQN and the start of the
interrupt service routine. During this time it is quite possible that
another interrupt with a higher priority occurs. It is the CIR that
contains the information that describes the interrupt source and its
priority. It is therefor recommended that the first operation upon
entering the interrupt service routine is the updating of the CIR.
(Recall that the contents of the GLOBAL registers reflect the content
of the CIR)
Summary
In the asynchronous mode all of the interface pins are usually used.
The synchronous mode usually will not use the IACKN and DACKN.
However there is no conflict in the quart if both modes are used in
the same application. (i.e. More than one device may control the
QUART) The principles to keep in mind are:
1. When IACKN is not used the CIR should be updated via
2. If DACKN is not used it should be disabled.
3. When in the asynchronous mode be sure DACKN is enabled.
4. With 68xxx type controllers the RDN signal must be generated.
command.
Product specification
SC26C94