Table 1-3. EZ-USB Series 2100 Pinouts by Pin Function
2125S
2126S
2135S
2136S
2121S
2131Q 2122S
2131S
2122T 2126T
Name
Type Default
Description
24
N/A
N/A
N/A
N/A
EA
Input
N/A External Access. If this signal is active (high), the
8051 fetches code from external memory instead of
the internal program RAM. If EA=0, the 8051
fetches code from external memory starting at
0x1B40 (AN2131).
19
20
68
8
9
8
9
9
9
XIN
Input
N/A Crystal Input. Connect this signal to a 12-MHz
series resonant, fundamental mode crystal and 22-
33-pF capacitor to GND. This pin may also be
driven by a 12-MHz clock.
10
34
10
34
XOUT
Output N/A Crystal Output. Connect this signal to a 12-MHz
series resonant, fundamental mode crystal and 22-
33-pF capacitor to GND. If XIN is driven by a 12-
MHz clock, this pin should not be connected.
N/A
N/A
PA0 or
T0OUT
I/O
I/O
I/O
I
Multiplexed pin whose function is selected by the
(PA0) T0OUT bit of the PORTACFG register. If T0OUT=0,
the pin is the bi-directional I/O port bit PA0. If
T0OUT=1, the pin is the active-high T0OUT signal
from 8051 Timer/Counter0.
T0OUT outputs a high level for one CLK24 clock
cycle when Timer0 overflows. If Timer0 is operated
in mode 3 (two separate timer/counters), T0OUT is
active when the low byte timer/counter overflows.
69
N/A
N/A
N/A
N/A
PA1 or
T1OUT
I
Multiplexed pin whose function is selected by the
(PA1) T1OUT bit of the PORTACFG register. If T1OUT=0,
the pin is the bi-directional I/O port bit PA1. If
T1OUT=1, the pin is the active-high T1OUT signal
from 8051 Timer-counter1
T1OUT outputs a high level for one CLK24 clock
cycle when Timer1 overflows. If Timer1 is operated
in mode 3 (two separate timer/counters), T1OUT is
active when the low byte timer/counter overflows.
70
N/A
N/A
N/A
N/A PA2 or OE#
I
Multiplexed pin whose function is selected by the OE
(PA2) bit of the PORTACFG register. If OE=0, the pin is
the bi-directional I/O port pin PA2. If OE=1, the pin
is an active-low output enable for external memory.
If the OE# pin is used, it should be externally pulled
up to Vcc to ensure that the write strobe is inactive
(high) at power-on.
Page 1-24
Chapter 1. Introducing EZ-USB
EZ-USB TRM v1.9
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