5
5
DESCRIPTION OF PIN FUNCTIONS
PIN NO.
NAME
SYMBOL
DESCRIPTION
23
nPulse 2
nPULSE2
Output. Network output. Outputs a synchronous
clock at 2x the data rate when backplane mode is
invoked.
24-31
P2.0-2.7
P2.0-2.7
Input/Output. Port 2 of the 8051. High order address
bus.
32
nProgram Store
Enable
nPSEN
Output.
33
Address Latch
Enable
ALE
Output.
34
Transmit Enable TXEN
Output. This signal is used to enable the drivers for
transmitting. The polarity of this signal is
programmable by grounding the nPULSE2 pin prior
to the POWER-UP.
nPULSE2 floating prior to the power-up = TXEN
active high
nPULSE2 grounded prior to the power-up = TXEN
active low. (This option is available only in the
Backplane mode).
35
nEnable
nEA
Input. When high, causes the 8051's outputs to tri-
state. When low, allows the 8051 to address external
memory. Must be low to execute code from the
embedded 8051.
36-43
P0.7-0.0
P0.7-0.0
Input/Output. Port 0 of the 8051. Multiplexed low
order address/data bus.
44
Power Supply
VCC
+5V power supply.
RESET CIRCUIT FOR THE COM20051
The power on reset circuit for the COM20051 should be designed to provide a clean, fast transition time
TTL input to the COM20051. Sufficient signal high time on RST (pin 10) should be provided after Vcc
reaches +5V DC. The following circuit, which provides an 8ms power-on reset pulse, is recommended:
Vcc (+5V)
22uF/
10V
RST (PIN 10)
220
74LS14
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