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AM79C985 Fiches technique(PDF) 20 Page - Advanced Micro Devices |
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AM79C985 Fiches technique(HTML) 20 Page - Advanced Micro Devices |
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20 / 48 page  20 Am79C985 PRELIMINARY Expansion Bus Interface The eIMR+ device expansion bus allows multiple eIMR+ devices to be interconnected. The expansion bus supports two modes of operation: internal arbitration mode and IMR+ mode. The internal arbitration mode uses a modified daisy-chain scheme to eliminate the need for any external arbitration cir- cuitry. The IMR+ mode maintains the full functionality of the IMR+ (Am79C981) expansion bus and benefits from minimum delays. In this mode, the eIMR+ device requires external circuitry to handle arbitration for con- trol of the bus. The eIMR+ arbitration mode is determined at reset. This occurs on the trailing edge of RST according to the state of SELI0-1, as illustrated in Figure 4. The eIMR+ device can be connected to a HIMIB device, as described in the eIMR+/HIMIB Interconnection sec- tion.The connection to a HIMIB device is not dependent on the mode of the expansion bus. In other words, the eIMR+ device can be connected to a HIMIB device whether the expansion bus is in internal-arbitration mode or IMR+ mode. Internal Arbitration Mode The internal arbitration mode uses a daisy-chain (cas- cade) configuration. SELI0-1 are arbitration inputs and SELO is the arbitration output. SELO goes LOW when there is activity on one or more of the eIMR+ ports, or a SELI input is LOW. The SEL lines are connected as shown in Figure 5. This technique allows activity indica- tion to propagate down the chain to the end device. All unused SELI inputs must be tied to VDD. ACK and COL are global activity I/O pins. When the eIMR+ device senses activity, it drives ACK LOW. Figure 4. Expansion Bus Mode Selection An eIMR+ device drives COL LOW when it senses more than one device is active; that is, if the device has an active port AND a SELI input is LOW, OR both SELI inputs are LOW. In Boolean notation, the formula for COL is as follows: COL = (Active port & (SELI1 + SELI0))+ (SELI1 & SELI0) where & represents the Boolean AND operation + represents the Boolean OR operation ACK and COL are mutually exclusive. If the eIMR+ de- vice driving ACK senses COL LOW, the device will deassert ACK. DAT and JAM are synchronized to CLK. DAT is the rep- etition of data from any connected port (either TP or AUI port) encoded in NRZ format. JAM is an internal collision indicator. If JAM is HIGH, the active eIMR+ de- vice has detected an internal collision across one or more of its ports. When this occurs, the DAT signal dis- tinguishes between single-port collisions and multiport collisions. DAT = 1 indicates a single port collision; DAT = 0 indicates a multiport collision. The drive capabilities of the I/O signals on the expan- sion bus (DAT, JAM, ACK, and COL) are sufficient to allow seven eIMR+ devices to be connected together without the use of external transceivers or buffers. The maximum number of eIMR+ devices that can be daisy chained is limited by the propagation delay of the eIMR+ devices. In practice, the depth of the cascade is limited to three eIMR+ devices, thus allowing a maxi- mum of seven eIMR+ devices connected together via this expansion bus as shown in Figure 5. The active device will not drive the data line, DAT, until one bit time (100 ns) after SELO goes LOW. This is to avoid a situation where two devices drive DAT simultaneously. IMR+ Mode In IMR+ mode, the expansion bus requires an external arbiter. The arbiter allows only one eIMR+ device to control the expansion bus. If more than one device at- tempts to take control, the arbiter terminates all access and signals a collision condition. In IMR+ mode, DAT and JAM retain the same function- ality as in internal arbitration mode, but ACK and COL are inputs to the eIMR+ device, driven by the external arbiter. The arbiter should drive ACK LOW when ex- actly one eIMR+ device is active. It should drive COL when more than one eIMR+ device is active. SELO is an output from the eIMR+ device. It indicates that the eIMR+ device has an active port and is requesting ac- cess to the bus. When ACK is HIGH, DAT and JAM are in the high-impedance state. DAT and JAM go active when ACK goes LOW. Refer to the Systems Applica- tions section (Fig.14) for the configuration of IMR+ mode of operation. Note: The IMR+ mode is recommended when arbitrating between multiple boards. RST SELI_0 Mode Selection SELI_1 SELI_0 Arbitration Mode X 1 Internal X 0 IMR+ 20651B-9 |
Numéro de pièce similaire - AM79C985 |
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Description similaire - AM79C985 |
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