W921E840A Datasheet, PDF (41/58 Pages) Winbond – 4-BIT MICROCONTROLLER

W921E840A/W921C840
6.14 Initial Condition Register of EPROM Program Method
There is one 4-bit of the initial condition register (not part of the RAM) in W921E840A to control the
micro-controller initial status after power-on. The format is described as following:
INI register: (initial value = 0FH)
b3
b2
b1
b0
0: f osc acts as RC oscillator type
1: fosc acts as crystal type
0: f1 = f L , f2 = f H _{only for W921E841A, 843A, 844A}
1: f1 = f H, f2 = f L
Reserved
Reserved
6.15 Reset
W921E840A/W921C840 provides two reset methods, pull low RESET pin and watch dog timer reset.
6.15.1 Reset by _RESET
256 machine
cycle
Program executed from
address 000H
Reset all
control reg.
As RESET pin is pulled low, system and all control registers are reset to initial state. After RESET
pin is in high level, system will delay 256 machine cycle time, then program is executed from address
000H.
6.15.2 Reset by Watch Dog Timer
STTM0.1
256 machine
cycle
Program executed from
address 000H
Reset all control
regs except
STTM0.1.
As watch dog timer underflows, the STTM0.1 is set, in the meantime, system and all control registers,
except data 1 in STTM0.1 bit is reserved, are reset to initial state, then after a delay of 256 machine
cycle time program is executed from address 000H. After system reset, user can detect STTM0.1 to
recognize which method of reset was used before.
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Publication Release Date: July 1999
Revision A3