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Handshake Solutions
HT80C51 User Manual
Reset
Page 18 of 132
© Philips Electronics N.V. 2005
3.
Reset
The reset input is the Z_R pin. An asynchronous reset is accomplished by holding the Z_R pin low.
The minimum low time is not depending on the clock frequency but it depends on the standard cell
library, placement and routing. However, usually a reset pulse of about 100ns is sufficient.
A reset initializes most of the SFRs. The following table lists the SFR reset values. The internal RAM
is not affected by reset. On power up the RAM content is not defined.
register
reset value
PC
ACC
B
PSW
SP
DPTR
PCON
IEN0
IEN1
IP0
IP1
TMOD
TCON
TH0
TL0
TH1
TL1
SCON
SBUF
POUT0
POUT1
POUT2
POUT3
S1CON
S1STA
S1DAT
S1ADR
SPCR
SPSR
SPDR
DCON
DKEY
DTXT
0000H
00H
00H
00H
07H
0000H
0xx0 000
00H
00H
00H
00H
00H
00H
00H
00H
00H
00H
00H
XX
FFH
FFH
FFH
FFH
00H
F8H
00H
00H
0000 0100
00H
00H
XX
XX
XX
Note:
“XX” means no initialization on reset.
[Table 1]
80C51 SFR Reset Values
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