4.3 TIMING SIGNALS

Three external timing elements are used by the PCL to time all operations:

IST is connected to OST via a short delay of about 5ns (1.0k and stray capacitance). The delay in the pin I/O drivers is the significant part of this delay. It is used for strobing internal latches and compensating for internal delays. This delay must be fairly symmetrical as both rising and falling edges are used.

ILT is connected to OLT via a longer delay which is used to compensate for parity generation and SS code generation delays and also for Write and Read access time to user registers. It should be set at the greater of the worst write or read access time for any user register, or 90ns. Both rising and falling edge delays are used. An RC integrator or similar delay device should be used. The maximum delay should be less than 1 microsecond; slower devices should use Busy (IBSY) or the WT line. If an RC network is used, low-bias diodes may be used to give separate adjustment of the rise and fall times. The rise time may then be reduced where parity has been disabled.

IUAT is connected to OUAT via a delay equal to that between the NTA increment signal and stabilized access to any user register. This includes NTA clock to OACA/OACB (max 50ns), the user's NTA decoder delay and the address access time of the user's registers. It does not have to include the read access time for the user's registers, which is compensated for by the long timer.

Table 1 gives typical values for the integrator components. These figures are guidelines only, based on the prototype performance. Delays in user designs must be tuned for correct operation and optimal performance. The figure 2pF shown in brackets refers to the input pin capacitance; no external capacitor is required.

• 4.3.1 Details of PCL timing