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Xl2000 development system Shenzhen, China Xuelin Electronics Co., Ltd.
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MCS-51 single-chip pin description
51 Series single-chips like 8031, 8051 and 89c51/89s51 are all adopting dual in-line
package(DIP) with 40Pins. The following are specifications of the 40pins chart. Power and
ground occupy two pins, external quartz oscillator clock occupy another two pins, total of
32(4*8bit) I/O ports occupy 32pins, the interrupt port multiplexed with P3 port. Next, let us
explain the functions of each pin one by one:
PIN20: ground.
PIN40: +5 V power supply.
PIN19: XTAL1 clock, on-chip oscillation circuit input.
PIN18: XTAL2 clock, on-chip oscillation circuit output.
8051 can work in two clock mode, one is the on-chip
clock oscillation mode, in this case, PIN18 and PIN19
need to be connected to an external quartz crystal
(2-12MHz) and oscillation capacitor. The oscillation
capacitor value takes 10p-30p. Another mode is the
external clock, XTAL1 connected to ground, external
clock signal connected to XTAL2.
Input and output (I / O) pins:
PIN39-PIN32: mapped to P0.0-P0.7 I/O port.
PIN1-PIN8: mapped to P1.0-P1.7 I/O port.
PIN21-PIN28: mapped to P2.0-P2.7 I/O port.
PIN10-PIN17: mapped to P3.0-P3.7 I/O port.
PIN9: RESET/Vpd reset signal. When power on
8051, clock circuit starts to work and a high TTL of
24 clock cycles signal occurs on RESET pin. By this
way, the system is reset and initialized.
The 8051 can be reset automatically and can also
be manually, see below. In addition, RESET/Vpd is a
multiplexed pin, during the Vcc power-down period, this pin can be connected to a
back-up power and make the RAM data inside the chip not lost.
PIN30: ALE/PROG, when the CPU accesses external program, ALE (address latch) used
to latch the output of low-bytes of the address. When CPU accesses internal memory,
ALE will emit a pulse signal of 1/6 clock cycle. This signal can be used to identify whether
the CPU works or not, and can also be used as an external clock output. When writing
EPROM, RROG pin will be used as input programming pulse.