TIMING CONSIDERATIONS

There are three general areas of timing that are important to consider when

designing with the transceiver: the actual time value for each is listed under the

“Performance Specifications” section of this document.

1. Start-up time

Start-up time is the time to transmitter or receiver readiness from the application

of power to the VIN pin on the PDN pin going high.

Transmitter readiness is defined as the presence of a fully locked carrier.

Receiver readiness is determined by valid data at the RXDATA pin. (This

assumes an incoming data stream.)

2. Mode turnaround time

This is the time taken by a powered-up module to switch between modes.

The RX-to-TX time is measured as the time from the TXEN pin going high and

the RXEN pin going low to the transmitter being ready to transmit data. Here

again, transmitter readiness is defined as the presence of a fully locked carrier.

The TX-to-RX time is measured as the time from the RXEN pin going high and

the TXEN pin going low to the receiver’s data output being valid. (This assumes

valid incoming data.)

It is important to note that TXEN and RXEN should never be high at the same

time. This will invalidate the timing parameters and may cause illegal emissions

from the transceiver.

3. Required transition interval.

This is the maximum amount of time that can elapse without a transition on the

TX data pin.

It is always important to think of data in both the analog and the digital domain.

Because the data stream is asynchronous and no particular format is imposed,

it is possible for the data to meet the baud-rate requirements of the module and

yet violate the analog frequency parameters. For example, if a 255 (0FF hex) is

being sent continuously the receiver would view the data as a DC level. The

receiver would hold that level until a transition was required to meet the minimum

frequency requirement. If no transition occurred, data integrity could not be

guaranteed. Thus, while no particular signal structure or code-balancing

requirement is imposed, the designer must insure that both analog and digital

signals have transitions suitable to meet the required transition interval

specification.

A TYPICAL SYSTEM EXAMPLE

To properly apply the transceiver, it is critical to understand and respect the

differences that exist between a wired and a wireless environment. At every

point in this system, there are timing and data-corruption issues that should be

understood and accounted for. The following section provides a basic outline of

the typical flow of events in a link incorporating SC-PA modules. While there are

many alternate methodologies, this section illustrates many of the considerations

previously mentioned.

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