CD1865 Datasheet, PDF (36/150 Pages) Intel Corporation – Intelligent Eight-Channel Communications Controller

CD1865 — Intelligent Eight-Channel Communications Controller
1. Provide three levels of interrupt support, with three separate levels and three separate vectors.
This is well-suited to Motorola^ 680X0 processors.
2. Provide a single level of interrupt support; this is an effective method when using 8-bit
processors such as the Z-80 and many Intel^
microprocessors.
3. Poll the device directly in software.
Once the host has ‘noticed’ the service request, it has the following two choices for acknowledging
the request and beginning to service it:
• Acknowledge the request by a hardware-based service acknowledgment, as is typically done
in interrupt-driven systems.
• Acknowledge the request in software by reading from a register in the CD1865.
Table 4. Service Request Methods
How the host detects the Service Request
1. Three-level
Hardware
Interrupt
2. Single-level
Hardware
Interrupt
3. Software
Polling
How the host
acknowledges the
Interrupt
a. Hardware-based
service
acknowledge
b. Software-based
service
acknowledge
1a
2a Not recommended
Full Interrupt – Type A (Inefficient)
3a Not recommended
(Inefficient)
1b
2b
Full Interrupt – Type B Single Interrupt
3b
Software Polled
Thus, there are six theoretically possible options for interfacing the CD1865 to the host system.
Two of the methods (2a and 3a) are not practical to implement without external hardware, and offer
no performance advantage. Each of the other four methods has advantages and drawbacks
depending on the type of host CPU being used and whether or not that host CPU supports more
than one CD1865. The four methods used are listed in Table 4.
• This method is called ‘Full Interrupt – Type A’. The system is fully interrupt driven with
acknowledgments in hardware. It requires a host with at least three interrupt priority levels
available and the ability to acknowledge on multiple levels. This is the technique used by
Motorola 680X0 processors. It is the most efficient method when the host CPU has a relatively
fast interrupt context switch time and when the host CPU has duties other than driving the
CD1865s.
• This method is called ‘Full Interrupt – Type B’. It still has three levels of interrupt, but
provides a single acknowledgment level. It is commonly used in Intel-type processor systems
where there is an 8259A interrupt controller. The 8259A receives the three levels of interrupt,
but it provides its own vector to the host rather than that of the CD1865s. Then the host
acknowledges the CD1865s Service Request by reading the Vector register.
• This method is called ‘Single Interrupt’, and is best-suited to systems having only a single
interrupt input, such as most 8-bit microprocessors. After the host receives its interrupt and is
entering its interrupt service routine, it reads the CD1865 to evaluate which of the three types
of service requests is responsible for the interrupt.Then it acknowledges the interrupt by
reading the appropriate Request Acknowledge register. Note that the single interrupt signal
must be generated by the logical OR of the three request outputs with external output gates,
not by ‘wire-OR’ing’ them.
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Datasheet