1. VMEbus Outline –Introduction –Electrical Characteristics –Mechanics –Functions –Data Transfer –Arbitration –Priority Interrupt Bus –Utilities Goal –Understand VMEbus fundamentals Reading –Microprocessor Systems Design, Clements, Ch. 10.3

2. Introduction VERSAbus -> Versa Module Europe bus -> VMEbus –developed for Eurocard in 1981 - popular PC card format –originally Motorola bus, now ANSI/IEEE/IEC standard Objectives –allow two cards to communicate independent of other cards –electrical and mechanical specification –protocol specification –terminology and definitions to precisely describe protocols –allow broad design latitute for cost/performance optimization »8, 16, 32-bit data widths, 24, 32-bit address widths –performance is device limited, rather than interface limited

3. Electrical Characteristics TTL compatible –standard TTL voltage levels –simple interfacing Bus size –≤ 21 slots across ≤ 500 mm –permits high speed Protection –inputs clamp negative excursions at ≤ -1.5V »TTL gates do this already –maximum driver short-circuit current specified Drive capability –AS*, DS0*, DS1* must have high current drive –bus arb, IACK can have low drive –330/470 ohm signal termination (equiv to 2.49V into 194 ohms) »can be used for open collector »floating lines go high

4. Mechanics Specifies single or double height cards –132.5 mm or 265.9 mm slots –100 mm by 160 mm or 233.35 mm by 160 mm boards Two connector system –96-pin connectors –primary connector P1 - provides all VMEbus functions »used on single-height cards »24-bit address, 16-bit data buses –secondary connector P2 »on double-height cards »use for 32-bit addresses, 32-bit data bus

5. Functions VMEbus is logically 4 separate buses –cards only need to implement parts they need Data transfer bus (DTB) –data, address, control lines for master to slave communication DTB arbitration bus –determines who will be DTB master Priority interrupt bus –7 level interrupts Utility bus –clock, reset, system fail, A/C fail lines

6. Bus Functional Modules DTB requestor –on board with master or interrupt handler –can request control of bus Interrupter –can generate interrupt request for master –can provide status information to interrupt handler on request Interrupt handler –can detect and handle interrupt requests DTB arbiter –receives DTB arbitration requests, prioritizes them, and grants bus to requestor DTB slave –responds to data transfer requests by master (e.g. memory) DTB master –can initiate bus transfers (e.g. CPU, DMA)

7. Data Transfer Bus Extension of 68000 asynchronous bus –but signals latched on clock –UDS*/LDS* -> DS1*/DS0* –LWORD* - 32-bit transfer –AM0-AM5 - address modifier »pass extra information to slave, e.g. FC0-FC2 from 68000 »memory map manipulation, address range »privilege levels Transfers –1-256 bytes at at time –address-only cycle –separate address and data strobes so master can send next address before it reads data

8. Arbitration Arbiter –in slot 1, determines master among requestors –BR0*-BR3* - bus request priority lines –arbiter can operate in round robin of priority –arbiter can grant to highest priority –single level priority - BR3* only »“daisy chain” modules so highest priority are closer to arbiter Signals –BGxOUT* - bus grant outputs –BGxIN* - bus grant inputs –used to daisy chain priority lines –BBSY* - busy, asserted by new master –BCLR* - clear, asserted by arbiter - give up bus

9. Priority Interrupt Bus 7-level prioritized vectored interrupts –use D0-D7 for interrupt ID Can have more than one interrupt handler device Signals –IRQ1-IRQ7 –IACK* –IACKIN* - daisy chain in –IACKOUT* - daisy chain out »makes sure only one interrupter gets acknowledgement »pass signal if not the requester at that interrupt level using interrupt acknowledge level on address bus

10. Utilities SYSCLK - system clock, 16 MHz –freerunning, no phase relationship to other clocks –can use for local clock generation SYSRESET* - system reset –open collector, from reset button, power monitor, etc. SYSFAIL* - system failure –cards keep low until they have powered up –assert when card detects failure ACFAIL* - A/C power supply has failed –cards can do power-down sequence