Introduction of XL2000 experimental
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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.
PIN29:
when accessing external program memory, this pin generates negative
pulse selection signal. PC's 16-bit data address will appear at the
P0 and P2 port. External program memory will put instruction at port
P0. It is read by the CPU and executated. PIN31:
EA/Vpp, internal and external memory selection pin. 8051 and 8751
single-chip has built-in 4KB code memory. When EA is high and
instruction address is less than 4KB, CPU loads instruction from
internal memory, if instruction address is greater than 4KB, CPU
loads instruction from external code memory. However, when EA is
low, CPU always loads instruction from external memory regardless of
address value. Icon
explanation:
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of each chapter | Source
program of experiment | Line
connect of the experiment |
SHEN ZHEN XUELIN ELECTRONIC TECHNOLOGY
CO.,LTD. | 
