PCA9517A(2008) Просмотр технического описания (PDF) - NXP Semiconductors.
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PCA9517A Datasheet PDF : 19 Pages
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NXP Semiconductors
PCA9517A
Level translating I2C-bus repeater
6. Functional description
Refer to Figure 1 “Functional diagram of PCA9517A”.
The PCA9517A enables I2C-bus or SMBus translation down to VCC(A) as low as 0.9 V
without degradation of system performance. The PCA9517A contains two bidirectional
open-drain buffers specifically designed to support up-translation/down-translation
between the low voltage (as low as 0.9 V) and a 3.3 V or 5 V I2C-bus or SMBus. All inputs
and I/Os are overvoltage tolerant to 5.5 V even when the device is unpowered (VCC(B)
and/or VCC(A) = 0 V). The PCA9517A includes a power-up circuit that keeps the output
drivers turned off until VCC(B) is above 2.5 V and the VCC(A) is above 0.8 V. VCC(B) and
VCC(A) can be applied in any sequence at power-up. After power-up and with the enable
(EN) HIGH, a LOW level on port A (below 0.3VCC(A)) turns the corresponding port B driver
(either SDA or SCL) on and drives port B down to about 0.5 V. When port A rises above
0.3VCC(A), the port B pull-down driver is turned off and the external pull-up resistor pulls
the pin HIGH. When port B falls first and goes below 0.3VCC(B) the port A driver is turned
on and port A pulls down to 0 V. The port B pull-down is not enabled unless the port B
voltage goes below 0.4 V. If the port B low voltage does not go below 0.5 V, the port A
driver will turn off when port B voltage is above 0.7VCC(B). If the port B low voltage goes
below 0.4 V, the port B pull-down driver is enabled and port B will only be able to rise to
0.5 V until port A rises above 0.3VCC(A), then port B will continue to rise being pulled up by
the external pull-up resistor. The VCC(A) is only used to provide the 0.3VCC(A) reference to
the port A input comparators and for the power good detect circuit. The PCA9517A logic
and all I/Os are powered by the VCC(B) pin.
6.1 Enable
The EN pin is active HIGH with an internal pull-up to VCC(B) and allows the user to select
when the repeater is active. This can be used to isolate a badly behaved slave on
power-up until after the system power-up reset. It should never change state during an
I2C-bus operation because disabling during a bus operation will hang the bus and
enabling part way through a bus cycle could confuse the I2C-bus parts being enabled.
The enable pin should only change state when the global bus and the repeater port are in
an idle state to prevent system failures.
6.2 I2C-bus systems
As with the standard I2C-bus system, pull-up resistors are required to provide the logic
HIGH levels on the buffered bus (standard open-collector configuration of the I2C-bus).
The size of these pull-up resistors depends on the system, but each side of the repeater
must have a pull-up resistor. This part designed to work with Standard mode and Fast
mode I2C-bus devices in addition to SMBus devices. Standard mode I2C-bus devices only
specify 3 mA output drive; this limits the termination current to 3 mA in a generic I2C-bus
system where Standard-mode devices and multiple masters are possible. Under certain
conditions higher termination currents can be used.
Please see Application Note AN255, I2C/SMBus Repeaters, Hubs and Expanders for
additional information on sizing resistors and precautions when using more than one
PCA9517A in a system or using the PCA9517A in conjunction with other bus buffers.
PCA9517A_2
Product data sheet
Rev. 02 — 5 May 2008
© NXP B.V. 2008. All rights reserved.
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