Rabbit 2000 - Early Memory Output-Enable Feature

218 Rabbit 2000 Microprocessor
B.2.13 Early Memory Output-Enable Feature
The early I/O enable feature was added to the Rabbit 2000C revision to relax the tight tim-
ing requirements for memory access when using the clock spectrum spreader. The early
I/O option extends the output enable time for the /OEx strobes and the write enable time
for the /WEx strobes by a half clock cycle. The Memory Timing Control Register
(MTCR) enables the extended timing for the memory output enable and write enable
strobes.
Memory read and write timing are discussed further in Chapter 15, “AC Timing Specifica-
tions.”
Table B-12. Memory Timing Control Register
Memory Timing Control Register (MTCR) (Address = 0x19)
Bit(s) Value Description
7:4 xxxx These bits are reserved and should not be used.
3 0 Normal timing for /OE1B (rising edge to rising edge, one clock minimum).
1 Extended timing for /OE1B (one-half clock earlier than normal).
2 0 Normal timing for /OE0B (rising edge to rising edge, one clock minimum).
1 Extended timing for /OE0B (one-half clock earlier than normal).
10
Normal timing for /WE1B (rising edge to falling edge, one and one-half clocks
minimum).
1
Extended timing for /WE1B (falling edge to falling edge, two clocks
minimum).
00
Normal timing for /WE0B (rising edge to falling edge, one and one-half clocks
minimum).
1
Extended timing for /WE0B (falling edge to falling edge, two clocks
minimum).