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AT90S8535 Datasheet(PDF) 24 Page - ATMEL Corporation

Part No. AT90S8535
Description  8-bit Microcontroller with 8K Bytes In-System Programmable Flash
Download  127 Pages
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Manufacturer  ATMEL [ATMEL Corporation]
Direct Link  http://www.atmel.com
Logo ATMEL - ATMEL Corporation

AT90S8535 Datasheet(HTML) 24 Page - ATMEL Corporation

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24
AT90S/LS8535
1041H–11/01
Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of 1 XTAL cycle dura-
tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period
t
TOUT. Refer to page 49 for details on operation of the Watchdog.
Figure 27. Watchdog Reset during Operation
MCU Status Register –
MCUSR
The MCU Status Register provides information on which reset source caused an MCU
reset.
• Bits 7..2 – Res: Reserved Bits
These bits are reserved bits in the AT90S8535 and always read as zero.
• Bit 1 – EXTRF: External Reset Flag
After a power-on reset, this bit is undefined (X). It can only be set by an External Reset.
A Watchdog Reset will leave this bit unchanged. The bit is cleared by writing a logical
zero to the bit.
• Bit 0 – PORF: Power-on Reset Flag
This bit is only set by a Power-on Reset. A Watchdog Reset or an External Reset will
leave this bit unchanged. The bit is cleared by writing a logical zero to the bit.
To summarize, Table 5 shows the value of these two bits after the three modes of reset.
To make use of these bits to identify a reset condition, the user software should clear
both the PORF and EXTRF bits as early as possible in the program. Checking the
PORF and EXTRF values is done before the bits are cleared. If the bit is cleared before
an External or Watchdog Reset occurs, the source of reset can be found by using Table
6.
Bit
7
6
543210
$34 ($54)
––
––––
EXTRF
PORF
MCUSR
Read/Write
R
RRRRR
R/W
R/W
Initial Value
0
0
0000
See Bit Description
Table 5. PORF and EXTRF Values after Reset
Reset Source
EXTRF
PORF
Power-on Reset
Undefined
1
External Reset
1
Unchanged
Watchdog Reset
Unchanged
Unchanged


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