Philips Semiconductors Preliminary data
87LPC762
Low power, low price, low pin count (20 pin)
microcontroller with 2 kbyte OTP
2001 Oct 26
9
FUNCTIONAL DESCRIPTION
Details of 87LPC762 functions will be described in the following
sections.
Enhanced CPU
The 87LPC762 uses an enhanced 80C51 CPU which runs at twice the
speed of standard 80C51 devices. This means that the performance of
the 87LPC762 running at 5 MHz is exactly the same as that of a
standard 80C51 running at 10 MHz. A machine cycle consists of 6
oscillator cycles, and most instructions execute in 6 or 12 clocks. A
user configurable option allows restoring standard 80C51 execution
timing. In that case, a machine cycle becomes 12 oscillator cycles.
In the following sections, the term “CPU clock” is used to refer to the
clock that controls internal instruction execution. This may
sometimes be different from the externally applied clock, as in the
case where the part is configured for standard 80C51 timing by
means of the CLKR configuration bit or in the case where the clock
is divided down via the setting of the DIVM register. These features
are described in the Oscillator section.
Analog Functions
The 87LPC762 incorporates two Analog Comparators. In order to
give the best analog function performance and to minimize power
consumption, pins that are actually being used for analog functions
must have the digital outputs and the digital inputs disabled.
Digital outputs are disabled by putting the port output into the Input
Only (high impedance) mode as described in the I/O Ports section.
Digital inputs on port 0 may be disabled through the use of the
PT0AD register. Each bit in this register corresponds to one pin of
Port 0. Setting the corresponding bit in PT0AD disables that pin’s
digital input. Port bits that have their digital inputs disabled will be
read as 0 by any instruction that accesses the port.
Analog Comparators
Two analog comparators are provided on the 87LPC762. Input and
output options allow use of the comparators in a number of different
configurations. Comparator operation is such that the output is a
logical one (which may be read in a register and/or routed to a pin)
when the positive input (one of two selectable pins) is greater than
the negative input (selectable from a pin or an internal reference
voltage). Otherwise the output is a zero. Each comparator may be
configured to cause an interrupt when the output value changes.
Comparator Configuration
Each comparator has a control register, CMP1 for comparator 1 and
CMP2 for comparator 2. The control registers are identical and are
shown in Figure 2.
The overall connections to both comparators are shown in Figure 3.
There are eight possible configurations for each comparator, as
determined by the control bits in the corresponding CMPn register:
CPn, CNn, and OEn. These configurations are shown in Figure 4.
The comparators function down to a V
DD
of 3.0V.
When each comparator is first enabled, the comparator output and
interrupt flag are not guaranteed to be stable for 10 microseconds.
The corresponding comparator interrupt should not be enabled
during that time, and the comparator interrupt flag must be cleared
before the interrupt is enabled in order to prevent an immediate
interrupt service.
BIT SYMBOL FUNCTION
CMPn.7, 6 — Reserved for future use. Should not be set to 1 by user programs.
CMPn.5 CEn Comparator enable. When set by software, the corresponding comparator function is enabled.
Comparator output is stable 10 microseconds after CEn is first set.
CMPn.4 CPn Comparator positive input select. When 0, CINnA is selected as the positive comparator input. When
1, CINnB is selected as the positive comparator input.
CMPn.3 CNn Comparator negative input select. When 0, the comparator reference pin CMPREF is selected as
the negative comparator input. When 1, the internal comparator reference V
ref
is selected as the
negative comparator input.
CMPn.2 OEn Output enable. When 1, the comparator output is connected to the CMPn pin if the comparator is
enabled (CEn = 1). This output is asynchronous to the CPU clock.
CMPn.1 COn Comparator output, synchronized to the CPU clock to allow reading by software. Cleared when the
comparator is disabled (CEn = 0).
CMPn.0 CMFn Comparator interrupt flag. This bit is set by hardware whenever the comparator output COn changes
state. This bit will cause a hardware interrupt if enabled and of sufficient priority. Cleared by
software and when the comparator is disabled (CEn = 0).
CMFn
SU01152
COnOEnCNnCPnCEn——
01234567
CMPn
Reset Value: 00h
Not Bit Addressable
Address: ACh for CMP1, ADh for CMP2
Figure 2. Comparator Control Registers (CMP1 and CMP2)
