Microchip Technology MCP3422, Manual

Page: 20 / 56

MCP3422/3/4
DS22088B-page 20
©
2008 Microchip Technology Inc.
FIGURE 5-1: Address Byte.
5.3.2 DEVICE ADDRESS BITS (A2, A1, A0)
AND ADDRESS SELECTION PINS
(
MCP3423
AND
MCP3424
)
The MCP3423 and MCP3424 have two external
device address pins (Adr1, Adr0). These pins can be
set to a logic high (or tied to V
DD
), low (or tied to V
SS
),
or left floating (not connected to anything, or tied to
V
DD
/2), These combinations of logic level using the
two pins allow eight possible addresses. Table 5-3
shows the device address depending on the logic sta-
tus of the address selection pins.
The device samples the logic status of the Adr0 and
Adr1 pins in the following events:
(a) Device power-up.
(b) General Call Reset
(See Section 5.4 “General Call” ).
(c) General Call Latch
(See Section 5.4 “General Call” ).
The device samples the logic status (address pins)
during the above events, and latches the values until a
new latch event occurs. During normal operation (after
the address pins are latched), the address pins are
internally disabled from the rests of the internal circuit.
It is recommended to issue a General Call Reset or
General Call Latch command once after the device
has powered up. This will ensure that the device reads
the address pins in a stable condition, and avoid latch-
ing the address bits while the power supply is ramping
up. This might cause inaccurate address pin detection.
When the address pin is left “floating”:
When the address pin is left “floating”, the address pin
momentarily outputs a short pulse with an amplitude of
about V
DD
/2 during the latch event. The device also
latches this pin voltage at the same time.
If the “floating” pin is connected to a large parasitic
capacitance (>20 pF) or to a long PCB trace, this short
floating voltage output can be altered. As a result, the
device may not latch the pin correctly.
It is strongly recommended to keep the “floating” pin
pad as short as possible in the customer application
PCB and minimize the parasitic capacitance to the pin
as small as possible (< 20 pF).
Figure 5-2 shows an example of the Latch voltage out-
put at the address pin when the address pin is left
“floating”. The waveform at the Adr0 pin is captured by
using an oscilloscope probe with 15 pF of capacitance.
The device latches the floating condition immediately
after the General Call Latch command.
FIGURE 5-2: General Call Latch
Command and Voltage Output at Address Pin
Left “Floating” (MCP3423 and MCP3424).
Start bit
Read/Write bit
Address Byte
R/W ACK
1 1 0
1
A2 A1 A0
Device Code Address Bits
(Note 1)
Address Byte:
Acknowledge bit
Address
Note 1:
MCP3423
and MCP3424: Configured by
the user. See Table 5-3 for address bit
configurations.
2: MCP3422: Programmed at the factory
during production.
Float waveform (output)
at address pin
SCL
SDA