System Design

In a device that processes 3 types of stream data, is a 3-port physical layer required?

No; the number of ports in the physical layer is unrelated to the number of connected devices or the number of stream data (number of isochronous transfer channels) that can be simultaneously processed.

Think of the number of ports as the number of connectors you wish to install in your device.
The number of stream data that can be simultaneously processed is determined by the specifications of the link layer controller.

If there are three physical layer ports, up to three devices can be directly connected. However, if the devices to be connected have multiple ports (i.e., 2 ports), three devices can be connected from there in a daisy chain, so only one physical layer port is required.

Note that even if these connections change, the total amount of data that can be processed remains the same. However, caution is required with the maximum data transfer rate of each physical layer.



Stream data in MPEG-TS or DV format is transmitted and received using the isochronous transfer method, but the number of channels that can be simultaneously processed in a single device depends on the specifications of the link layer controller.

For example, with NEC Electronics' OHCI controller, four channels can be simultaneously processed, and with our AV-Link family, the number is two (uPD72891, 72893) or five (MC-10024).

Note that the number of channels that can be processed by the entire IEEE1394 bus is restricted by the total bandwidth.

When using a 4-pin connector, what should I do about the ground connection?

A chassis ground pin is not assigned in a 4-pin connector.
Four-pin connectors are designed and standardized for devices in which the ground is floating, such as video camcorders and other battery-powered devices.
The chassis of a 4-pin connector is defined as GND (equivalent to VG in a 6-pin connector), so the chassis must be directly connected to GND on the board (AGND).

Due to this ground connection, design your device assuming that the ground of the physical layer in the communicating device will be the same potential.

When using a 4-pin connector, what should I do about the CPS and PC[0:2] settings?

With a 4-pin connector, only the TpA/TpB signals are connected to the communicating device, so information on the power supply is not required. Accordingly, the CPS (Cable Power Status) and PC (Power Class) signals are usually handled as follows.

<Example of uPD72852> CPS: Pulled down by 100 kΩ, PC[0:2]: Individually pulled down by 10 kΩ



With this setting in a 4-pin connector, the device is recognized as a device that does not receive power from an external device or higher-level application.

What cautions do I have to take when the connector is on a different board to the physical layer board?

When the board on which the physical layer is mounted is separate from the board on which the connector is mounted, you must take care with how the ground is wired.
Separate boards are connected by a cable, and their grounds must be strongly connected by this cable.

If other components are mounted on the same board as the connector, ensure that the connector ground is separated from other grounds and that it has the same potential as the ground of the physical layer board connected by the cable.

There are various higher-level IEEE1394 protocols; how are they implemented?

As you mentioned, the IEEE1394 standard covers various higher-level protocols.
For each protocol specification, the packet data configuration and the operation of each address to/from which packets are sent/received is prescribed.
In other words, protocol specifications are implemented by sending/receiving data with a specific configuration via a unique address.

In NEC Electronics products, the form in which the protocol is implemented differs depending on the type of link layer.
With an OHCI controller or the MC-10024, the way in which each protocol is implemented is defined by the higher-level application, so this is developed by the customer.

With the uPD72891 and 72893, a protocol layer commonly known as an AV protocol is incorporated in the product, so it is necessary for you to implement control of this protocol in your application.

What is the procedure for starting up an IEEE1394 LSI?

It depends on the design specifications of the device, but when there are multiple connectors installed, by supplying power to just the physical layer in the power supply standby status, the physical layer can be made to function as a repeater.

Accordingly, communication can be established between the devices connected via the IEEE1394 device.

Then by applying main power to the IEEE1394 device and controlling the link layer LPS (Link Power Status) pin via a register setting as soon as the link layer has been initialized, the fact that the link layer is active can be reported to the physical layer.

Upon detection of the signal by the LPS pin, the physical layer then makes the PHY-Link interface active, just as supplying of the clock required to operate the link layer is started.



If reset of the physical layer is released at the same time as reset of the link layer, usually more time is required to start up the physical layer.
This is because it takes some time for the physical layer clock to stabilize.

NEC Electronics' physical layer family incorporates a function that starts supplying the clock to the link layer once a stable clock can be supplied.

I'm designing an IDE bridge. When should the IEEE1394 LSI be reset?

Basically, an IEEE1394 system should only be reset when power is applied. In a bridge application that includes an IEEE1394 interface, each interface is initialized asynchronously.

In general, the IEEE1394 LSI does not need to be reset even when interfaces other than the IEEE1394 interface are reset.
Rather, resetting the IEEE1394 LSI is not desirable because in this case communication is cut off for the amount of time it takes to execute re-initialization.

Note that data transferred between the IEEE1394 interface and other interfaces may be cleared due to a FIFO reset, etc., as required.
In this case also, action may be taken such as reporting to another IEEE1394 device that the device containing the IEEE1394 LSI has been reset by BUS_RESET, etc., without resetting the IEEE1394 LSI.

These actions differ depending on the application, so a reset procedure that is ideal for each application is required.

The physical layer and link layer can be connected directly or via an isolation barrier.
How can I judge which is the most effective connection?

The isolation barrier connection (AC connection) is incorporated in the IEEE1394 standard in order to prevent ground loops occurring between the devices connected to the IEEE1394 interface.

If the devices are connected via 6-pin connectors and the grounds of the devices have the same potential as the primary power supply ground (not the assumed ground), this potential can be maintained by floating the transfer path with the communicating device.

Consequently, an isolation barrier connection is very effective for preventing a degradation in communication quality when there is a difference in potential at each ground.

In this case, both the interface signals and the physical layer's power supply must be isolated.
In a device with a plastic chassis or in which a ground is only defined for a secondary power supply, the ground with the communicating device is made the same potential via the cable connection, so a direct connection, which involves mounting fewer components, is recommended.

Why is a choke coil required in common mode?

A choke coil is a very useful component for improving the VCCI/EMI characteristics.
The maximum effect can be gained by positioning the choke coil as close as possible to the connector.

A choke coil also has a secondary effect of slightly blunting the S200/S400 speed signals, making it easy to detect them on the receiving side.

Be sure to select a choke coil with the correct impedance characteristics, in accordance with the maximum transfer rate of the physical layer used.

Caution The pin layout of the choke coil may differ depending on the manufacturer. Be sure to check this when selecting components and making circuit diagrams.

How can I maintain signal quality in IEEE1394 communication?

In general, the IEEE1394 signal quality can be improved by observing the following points.

(1) Reduce unnecessary reflection
This can be done by positioning the physical layer and terminating resistor as close together as possible.

(2) Stabilize the ground (keep it at the same potential, in terms of AC and DC, as the ground of the communicating device)
To do this, it is necessary to exercise caution in the circuits and layout design near the connector and between the connector and the physical layer.

(3) Supply stable power to the physical layer
The power consumption of the physical layer is not constant; power is consumed in repeated spurts. These spurts can be effectively absorbed by positioning an electrolytic capacitor, etc., as close as possible to the physical layer.

Is there a procedure for efficiently evaluating hardware that incorporates an IEEE1394 product?

The most effective first step is to check the operation of the physical layer.
Connect an evaluation device such as a bus analyzer and check whether a normal SelfID_Packet can be transmitted when the cable is connected.

Next, insert the evaluation device between the two communicating devices and check the repeat operation of the physical layer.
You'll need to check that the bias is not fluctuating and that the waveform has not lost shape.

If everything is normal at this point, you should evaluate the link layer and above.
The procedure for evaluating the link layer differs significantly depending on the product, so please contact your local sales office for details.