83C576 Datasheet, PDF (18/46 Pages) NXP Semiconductors – 80C51 8-bit microcontroller family 8K/256 OTP/ROM, 6 channel 10-bit A/D, 4 comparators, failure detect circuitry, watchdog timer

Philips Semiconductors
80C51 8-bit microcontroller family
8K/256 OTP/ROM, 6 channel 10-bit A/D, 4 comparators,
failure detect circuitry, watchdog timer
Product specification
83C576/87C576
The tap selection is programmable. The watchdog main counter is a
down counter clocked (decremented) each time the programmable
prescaler overflows. The watchdog generates an underflow signal
(and is autoloaded) when the watchdog is at count 0 and the prescaler
clock decrements the watchdog. The watchdog is 8 bits long and the
autoload value can range from 0 to FFH. (The autoload value of 0 is
permissible since the prescaler is cleared upon autoload).
This leads to the following user design equations. Definitions :t_OSC
is the oscillator period, N is the selected prescaler tap value, W is
the main counter autoload value, t_MIN is the minimum watchdog
time-out value (when the autoload value is 0), t_MAX is the maximum
time-out value (when the autoload value is FFH), t_D is the design
time-out value.
t_MIN = t_OSC × 2 × 64
t_MAX = t_MIN × 128 × 256
t_D = t_MIN × 2^PRESCALER × (W + 1)
(where prescaler = 0, 1, 2, 3, 4, 5, 6, or 7)
Note that the design procedure is anticipated to be as follows. A
t_MAX will be chosen either from equipment or operation
considerations and will most likely be the next convenient value
higher than t_D. (If the watchdog were inadvertently to start from 00H,
an underflow would be guaranteed, barring other anomalies, to
occur within t_MAX).
The software must be written so that a feed operation takes place
every t_D seconds from the last feed operation. Some tradeoffs may
need to be made. It is not advisable to include feed operations in
minor loops or in subroutines unless the feed operation is a specific
subroutine.
Watchdog Control Register (WDCON)
Address C4H
The following bits of this register are read only in the ROM part
when EA is high: WDMOD, DPD, OFRE, LVRE, PRE0, PRE1, and
PRE2. That is, the register will reflect the mask programmed values.
In the ROM part with EA high, these bits are taken from mask coded
bits and are not readable by the program. WDRUN is read only in
the ROM part when EA is high and WDMOD is in the watchdog
mode. When WDMOD is in the timer mode, WDRUN functions
normally.
The parameters written into WDMOD, DPD, OFRE, LVRE, PRE0,
PRE1, and PRE2 by the program are not applied directly to the
watchdog timer subsystem. The watchdog timer subsystem is
directly controlled by a second register which stores these bits. The
transfer of these bits from the user register to the second control
register takes place when the watchdog is fed. This prevents
random code execution from directly foiling the watchdog function.
This does not affect the operation where these bits are taken from
mask coded values.
The reset values of the WDCON and WDL registers will be such that
the timer resets to the watchdog mode with a timeout period of 2 ×
64 × 128 × t_OSC. The watchdog timer does not generate an interrupt.
Additional bits in WDCON are used to disable reset generation by
the oscillator fail and low voltage detect circuits. WDCON can be
written by software only by executing a valid watchdog feed
sequence.
WDCON Register Bit Definitions
WDCON.7 PRE2
Prescaler Select 2, reset to 1
WDCON.6 PRE1
Prescaler Select 1, reset to 1
WDCON.5 PRE0
Prescaler Select 0, reset to 1
WDCON.4 LVRE
Low Voltage Reset Enable, reset to 1
(enabled)
WDCON.3 OFRE Oscillator Fail Reset Enable, reset to 1
(enabled)
WDCON.2 DPD
Disable Power Down
WDCON.1 WDRUN Watchdog Run, reset to 1 (enabled)
WDCON.0 WDMOD Watchdog Mode, reset to 1 (watchdog
mode)
Enhanced UART
The UART operates in all of the usual modes that are described in
the first section of this book for the 80C51. In addition the UART can
perform framing error detect by looking for missing stop bits, and
automatic address recognition. The 8XC576 UART also fully
supports multiprocessor communication as does the standard
80C51 UART.
When used for framing error detect the UART looks for missing stop
bits in the communication. A missing bit will set the FE bit in the
SCON register. The FE bit shares the SCON.7 bit with SM0 and the
function of SCON.7 is determined by PCON.6 (SMOD0) (see Figure
20). If SMOD0 is set then SCON.7 functions as FE. SCON.7
functions as SM0 when SMOD0 is cleared. When used as FE
SCON.7 can only be cleared by software. Refer to Figure 19.
The serial port transmitter data can be inverted by setting the TXI
(AUXR.2) bit. For normal operation, the TXI bit should be cleared.
Automatic Address Recognition
Automatic Address Recognition is a feature which allows the UART
to recognize certain addresses in the serial bit stream by using
hardware to make the comparisons. This feature saves a great deal
of software overhead by eliminating the need for the software to
examine every serial address which passes by the serial port. This
feature is enabled by setting the SM2 bit in SCON. In the 9 bit UART
modes, mode 2 and mode 3, the Receive Interrupt flag (RI) will be
automatically set when the received byte contains either the “Given”
address or the “Broadcast” address. The 9 bit mode requires that
the 9th information bit is a 1 to indicate that the received information
is an address and not data. Automatic address recognition is shown
in Figure 21.
The 8 bit mode is called Mode 1. In this mode the RI flag will be set
if SM2 is enabled and the information received has a valid stop bit
following the 8 address bits and the information is either a Given or
Broadcast address.
Mode 0 is the Shift Register mode and SM2 is ignored.
1998 Jun 04
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