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82077SL: t23a Timing Clarification

The t23a timing is a new timing introduced in the 82077SL data sheet. It applies to systems that use DMA data transfer cycles under certain operating conditions. T23a is the minimum time from DMA request (DRQ) active to DMA acknowledge (DACK#) inactive. Most ISA/EISA/MCA systems have large values for the t23a timing and will not violate this timing. However, systems that incorporate ASIC or proprietary DMA controllers will have to ensure that this specification is met. This timing is applicable to all versions of the 82077 controllers.

Further Clarification of Condition

Figure 1 shows t23a in a DMA cycle timing diagram. This specification includes DMA latency time in the system. This timing must be met only during the following conditions in the system:

the FIFO is disabled
the DMA is performing single byte mode transfers
a write cycle is being executed

Systems that have the FIFO enabled or are using demand mode DMA transfers do not need to observe this timing specification. The falling edge of the DACK# removes the DRQ. However, the internal sampling circuitry of the 82077SL requires at least t23a time to ensure that a valid DACK# has been asserted. If the system is in violation of the t23a timing the 82077SL may raise an additional DRQ before the previous DACK# has been acknowledged and result in a mismanaged DMA transfer.

The worst case timing for t23a is at the 250 Kbps data rate. At this data rate, the minimum time required from the active edge of the DRQ to when the DACK# signal is inactive is 333 ns. Note that t23a includes DMA latency. Systems using 82077SL must allow at least 333 ns for t23a timing provided the system is running a single byte DMA write cycle with the FIFO disabled. If the system can not meet this timing, the problem can be solved by enabling the FIFO with a threshold of one. This allows the design to retain the current DMA scheme and meet the specification.

Figure 1. t23a in a DMA timing
Figure 1. t23a in a DMA timing