Z90251 Datasheet, PDF (24/97 Pages) Zilog, Inc. – eZVision 200 Television Controllers with OSD

Z90255 ROM and Z90251 OTP
32 KB Television Controller with OSD
16
Stop-Mode Recovery. After this point, the register cannot be modified by any
means, intentional or otherwise.
The WDT is permanently enabled after Reset. To ensure that the WDT is set
properly, use the following instructions as the first two instructions:
DI
WDT
The Watch-Dog timer must then be constantly refreshed within the required
timeout by executing the WDT Instruction.
Note: Executing the WDT instruction affects the Z (zero), S (sign),
and V (overflow) flags.
A system reset overrides all other operating conditions and puts the micro-
controller into a known state. To initialize the chipÕs internal logic, the Reset input
must be held Low for at least 5 XTAL clock cycles. The control registers and ports
are reset to default conditions after a POR, a reset from the Reset pin, or a WDT
timeout while in Run Mode and Halt Mode. The control registers and ports are not
reset to their default conditions after Stop Mode Recovery and WDT timeout while
in Stop Mode.
The program counter is loaded with 000Ch. I/O ports and control registers are
configured to their default reset states.
Resetting the microcontroller does not Affect the contents of the general-purpose
registers.
The Watch-Dog Timer (WDT) is a retriggerable, one-shot timer that resets the
microcontroller if it reaches its terminal count. When operating in the Run, Halt or
Stop Modes, a WDT reset is functionally equivalent to a hardware POR reset.
4 Stop Mode and Halt Mode Operation
4.1 Power-Down Halt-Mode Operation
The Halt Mode suspends instruction execution and turns off the internal CPU
clock. The on-chip oscillator circuit remains active so the internal clock continues
to run and is applied to the counter/timer(s) and interrupt logic.
To enter the Halt Mode, the instruction pipeline must be flushed first to avoid
suspending execution in mid-instruction. To do this, the application program must
PS001301-0800