56
CHAPTER 3 CPU
I
Mode Pins
The MB89570 series devices are single-chip mode devices. The mode pins (MOD1 and MOD0)
must be tied to VSS. The mode pin settings determine whether the mode data and reset vector
are read from internal ROM.
Do not change the mode pin settings, even after the reset has completed.
I
Mode Fetch
When the CPU wakes up from a reset, the CPU reads the mode data and reset vector from
internal ROM.
H
Mode data (address: FFFD
H
)
Always set the mode to "00
H
" (single-chip mode).
H
Reset vector (address: FFFE
H
(upper), FFFF
H
(lower))
Contains the address where execution is to start after completion of the reset. The CPU starts
executing instructions from the address contained in the reset vector.
I
Oscillation Stabilization Delay Reset State
On products with power-on reset, the reset operation for a power-on reset or external reset in
subclock or stop (main/sub) mode starts after the main clock oscillation stabilization delay time
selected by the stabilization delay time option. If the CPU has not woken up from the external
reset input when the delay time completes, the reset operation does not start until the CPU
wakes up from external reset.
As the oscillation stabilization delay time is also required when an external clock is used, a reset
requires that the external clock is input.
The main clock oscillation stabilization delay time is timed by the timebase timer.
On products without power-on reset, the oscillation stabilization delay reset state is not used.
Therefore, for such products, hold the external reset pin (RSTX) at the "L" level to disable the
CPU operation until the source oscillation stabilizes.
I
Effect of Reset on RAM Contents
The contents of RAM are unchanged before and after a reset other than power-on reset. If an
external reset is input close to a write timing, however, the contents of the write address cannot
be assured. For this reason, all RAM locations being used should be initialized following reset.