DP83901AV datasheet(45/62 Pages) NSC

45 / 62 page

130 Bus Arbitration and Timing (Continued)

SLAVE MODE TIMING

When CS is low the SNIC becomes a bus slave The CPU

can then read or write any internal registers All register

access is byte wide The timing for register access is shown

below The host CPU accesses internal registers with four

address lines RA0 – RA3 SRD and SWR strobes

ADS0 is used to latch the address when interfacing to a

multiplexed address data bus Since the SNIC may be a

local bus master when the host CPU attempts to read or

write to the controller an ACK line is used to hold off the

CPU until the SNIC leaves master mode Some number of

BSCK cycles is also required to allow the SNIC to synchro-

nize to the read or write cycles

Write to Register

TLF10469 – 30

Read from Register

TLF10469 – 31

TIME BETWEEN CHIP SELECTS

The SNIC requires that successive chip selects be no closer

than 4 bus clocks (BSCK) together If the condition is violat-

ed the SNIC may glitch ACK CPUs that operate from pipe-

lined instructions (ie 386) or have a cache (ie 486) can

execute consecutive IO cycles very quickly The solution is

to delay the execution of consecutive IO cycles by either

breaking the pipeline or forcing the CPU to access outside

its cache

Time between Chip Selects

TLF10469 – 58

DP83901AV データシート(PDF) 45 Page - National Semiconductor (TI)