87C575 Philips Semiconductors, 87C575 Datasheet - Page 9

• Manufacturer Part Number: 87C575
• Description: 80C51 8-bit microcontroller family 8K/256 OTP/ROM/ROMless/ 4 comparator/ failure detect circuitry/ watchdog timer
• Manufacturer: Philips Semiconductors

Philips Semiconductors
POWER ON CLEAR/
POWER ON FLAG
An on-chip Power On Detect Circuit resets
the 8XC575 and sets the Power Off Flag
(PCON.4) on power up or if V
zero momentarily. The POF can only be
cleared by software. The RST pin is not
driven by the power on detect circuit. The
POF can be read by software to determine
that a power failure has occurred and can
also be set by software.
LOW VOLTAGE DETECT
An on-chip Low Voltage Detect circuit sets
the Low Voltage Flag (PCON.3) if V
below V
Characteristics) and resets the 8XC575 if the
Low Voltage Reset Enable bit (WDCON.4) is
set. If the LVRE is cleared, the reset is
disabled but LVF will still be set if V
The RST pin is not driven by the low voltage
detect circuit. The LVF can be read by
software to determine that V
LVF can be set or cleared by software.
OSCILLATOR FAIL DETECT
An on-chip Oscillator Fail Detect circuit sets
the Oscillator Fail Flag (PCON.5) if the
oscillator frequency drops below OSCF for
one or more cycles (see AC Electrical
Characteristics: OSCF) and resets the
8XC575 if the Oscillator Fail Reset Enable bit
(WDCON.3) is set. If OFRE is cleared, the
reset is disabled but OSF will still be set if the
oscillator fails. The RST pin is not driven by
the oscillator fail detect circuit. The OSF can
be read by software to determine that an
oscillator failure has occurred. The OSF can
be set or cleared by software.
1998 May 01
80C51 8-bit microcontroller family
8K/256 OTP/ROM/ROMless, 4 comparator, failure detect circuitry, watchdog timer
LOW
REFERENCE)
(LOW V
VLOW
(see DC Electrical
CC
V
CC
RST
CC
POWER-ON DETECT
+
–
CC
was low. The
drops to
CC
CC
WATCHDOG FEED
is low.
drops
OSC FREQ BELOW OSCF
(MIN FREQUENCY)
SMOD1
PRE2
LOW ACTIVE RESET
One of the most notable features on this part
is the low active reset. At this time this is the
only 80C51 derivative available that has low
active reset. This feature makes it easier to
interface the 8XC575 into an application to
accommodate the power-on and low voltage
conditions that can occur. The low active
reset operates exactly the same as high
active reset with the exception that the part is
put into the reset mode by applying a low
level to the reset pin. For power-on reset it is
also necessary to invert the power-on reset
circuit; connecting the 8.2K resistor from the
reset pin to V
the reset pin to ground. Figure 1 shows all of
the reset related circuitry.
When reset the port pins on the 87C575 are
driven low asynchronously. This is different
from all other 80C51 derivatives.
The 8XC575 also has Low voltage detection
circuitry that will, if enabled, force the part to
reset when V
level. Low Voltage Reset is enabled by a
normal reset. Low Voltage Reset can be
disabled by clearing LVRE (bit 4 in the
WDCON SFR) then executing a watchdog
feed sequence (A5H to WFEED1 followed
immediately by 5A to WFEED2). In addition
there is a flag (LVF) that is set if a low voltage
condition is detected. The LVF flag is set
even if the Low Voltage detection circuitry is
disabled. Notice that the Low voltage
detection circuitry does not drive the RST#
pin so the LVF flag is the only way that the
microcontroller can determine if it has been
reset due to a low voltage condition.
SHADOW REGISTER
SMOD0
PRE1
FOR WDCON
Figure 1. Reset Circuitry
CC
CC
PRE0
OSF
(on the part) fails below a set
and the 10 f capacitor from
9
LVRE
POF
OFRE
LVF
WDRUN
GF0
The 8XC575 has an on-chip power-on
detection circuit that sets the POF (PCON.4)
flag on power up or if the V
momentarily drops to 0V. This flag can be
used to determine if the part is being started
from a power-on (cold start) or if a reset has
occurred due to another condition (warm
start).
TIMERS
The 87C575 has four on-chip timers.
Timers 0 and 1 are identical in every way to
Timers 0 and 1 on the 80C51.
Timer 2 on the 8XC575 is identical to the
80C52 Timer 2 (described in detail in the
80C52 overview) with the exception that it is
an up or down counter. To configure the
Timer to count down the DCEN bit in the
T2MOD special function register must be set
and a low level must be present on the T2EX
pin (P1.1).
The Watchdog timer operation and
implementation is the same as that for the
8XC550 (described in the 8XC550 overview)
with the exception that the reset values of the
WDCON and WDL special function registers
have been changed. The changes in these
registers cause the watchdog timer to be
enabled with a timeout of 98304
when the part is reset. The watchdog can be
disabled by executing a valid feed sequence
and then clearing WDRUN (bit 2 in the
WDCON SFR).
WDTOF
GF1
WDMOD WDCON
IDL
80C575/83C575/
PCA WATCHDOG
WATCHDOG TIMER
(87GH)
PCON
(C0H)
Product specification
CC
8xC575
INTERNAL
RESET
87C575
SU00239
level
T
OSC