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MC68340 データシート(PDF) 81 Page - Freescale Semiconductor, Inc |
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MC68340 データシート(HTML) 81 Page - Freescale Semiconductor, Inc |
81 / 441 page 3- 32 MC68340 USER’S MANUAL MOTOROLA 3.5 BUS EXCEPTION CONTROL CYCLES The bus architecture requires assertion of DSACK ≈ from an external device to signal that a bus cycle is complete. Neither DSACK ≈ nor AVEC is asserted in the following cases: • DSACK ≈/AVEC is programmed to respond internally. • The external device does not respond. • Various other application-dependent errors occur. The MC68340 provides BERR when no device responds by asserting DSACK ≈/AVEC within an appropriate period of time after the MC68340 asserts AS. This mechanism allows the cycle to terminate and the MC68340 to enter exception processing for the error condition. HALT is also used for bus exception control. This signal can be asserted by an external device for debugging purposes to cause single bus cycle operation, or, in combination with BERR, a retry of a bus cycle in error. To properly control termination of a bus cycle for a retry or a bus error condition, DSACK ≈, BERR, and HALT can be asserted and negated with the rising edge of the MC68340 clock. This assures that when two signals are asserted simultaneously, the required setup and hold time for both is met for the same falling edge of the MC68340 clock. This or an equivalent precaution should be designed into the external circuitry to provide these signals. Alternatively, the internal bus monitor could be used. The acceptable bus cycle terminations for asynchronous cycles are summarized in relation to DSACK ≈ assertion as follows (case numbers refer to Table 3-4): • Normal Termination: DSACK ≈ is asserted; BERR and HALT remain negated (case 1). • Halt Termination: HALT is asserted at the same time as or before DSACKx, and BERR remains negated (case 2). • Bus Error Termination: BERR is asserted in lieu of, at the same time as, or before DSACK ≈ (case 3) or after DSACK≈ (case 4), and HALT remains negated; BERR is negated at the same time as or after DSACK ≈. • Retry Termination: HALT and BERR are asserted in lieu of, at the same time as, or before DSACK ≈ (case 5) or after DSACK≈ (case 6); BERR is negated at the same time as or after DSACK ≈, and HALT may be negated at the same time as or after BERR. Table 3-4 lists various combinations of control signal sequences and the resulting bus cycle terminations. To ensure predictable operation, BERR and HALT should be negated according to the specifications given in Section 11 Electrical Characteristics. DSACK ≈ BERR, and HALT may be negated after AS. If DSACK ≈ or BERR remain asserted into S2 of the next bus cycle, that cycle may be terminated prematurely. EXAMPLE A: A system uses a bus monitor timer to terminate accesses to an unpopulated address space. The timer asserts BERR after timeout (case 3). Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com |
同様の部品番号 - MC68340 |
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同様の説明 - MC68340 |
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