UPD784020 Datasheet, PDF (53/90 Pages) NEC – 16/8-BIT SINGLE-CHIP MICROCOMPUTER

mPD784020, 784021
12. RESET FUNCTION
Applying a low-level signal to the RESET pin initializes the internal hardware (reset status).
When the RESET input makes a low-to-high transition, the following data is loaded into the program counter (PC):
• Eight low-order bits of the PC : Contents of location at address 0000H
• Intermediate eight bits of the PC : Contents of location at address 0001H
• Four high-order bits of the PC : 0
The PC contents are used as a branch destination address. Program execution starts from that address. Therefore,
a reset start can be performed from an arbitrary address.
The contents of each register can be set by software, as required.
The RESET input circuit contains a noise eliminator to prevent malfunctions caused by noise. This noise eliminator
is an analog delay sampling circuit.
Fig. 12-1 Accepting a Reset
RESET
(input)
Delay
Delay
Delay
Execute instruction
Initialize PC _{at reset start address}
Internal reset signal
Start reset
End reset
For power-on reset, the RESET signal must be held active until the oscillation settling time (approximately 40 ms)
has elapsed.
Fig. 12-2 Power-On Reset
Oscillation settling time
Delay Initialize PC
Execute instruction at
reset start address
VDD
RESET
(input)
Internal reset signal
End reset
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