Micrel, Inc.

MICRF302

March 2010

8

M9999-032610-A

MICRF302 Parallel Encoder Features

Transmit Timeout

The MICRF302 implements a 25-second (nominal)

transmit timeout timer. If a user pushes an MICRF302

button and holds it down, transmission of packets (in

repeated groups of four) will continue for no more than

25 seconds. Afterwards, the MICRF302 shuts down and

waits to be restarted by another button push. The button

must be released before it can be pushed again to

restart packet transmission.

The purpose of the transmit timeout is to prevent battery

drain and unnecessary transmission where a user might,

for example, sit on a remote control.

Preamble/Dead-Time/Sync Format

The MICRF302 sends a lengthy amount of information at

the start of the packet to help the decoder synchronize to

the incoming data stream. The format is as followed:

1. 32 zeros

2. Three-bit dead time of no Manchester-coded

data. This allows the MICRF302 to be used with

receivers whose AGC benefits from the

presence of a dead time.

3. Four ones (1111) mark the end of the preamble,

dead time, and sync. The bit following the last

one of the four is the first data bit of the 20-bit

DEVADR field.

Inter-Packet Delay

The delay (dead time, no Manchester-coded data)

between subsequent packets is always eight bit-times.

Important: Encoder Bit-Time Variation

The frequency of the MICRF302’s on-chip oscillator

varies by ±10% over supply voltage, temperature, and

manufacturing tolerances. It is important to remember

that this means that the MICRF302 Encoder’s bit time

does not vary by ±10%; it varies as follows. Consider an

MICRF302 Encoder set to transmit at a nominal rate of

1 kHz, with a nominal bit-time of 1 ms. The transmit

frequency will then fall in the range of 900 Hz to 1.1 kHz.

The corresponding bit-times are 1.111 ms to 0.909 ms.

This actually corresponds to a variation in bit-time of –

9.1% to +11.1%. It is critical that the decoder

accommodate incoming data streams whose bit-times

fall within this range, not ±10%.

TX Enable

The MICRF302 TXEN is normally low and goes active-

high ~ 40 ms before data is seen at the output (Figure 7,

8) and remains high until D0-D3 is deactivated or the

internal MICRF302 time out has elapsed (Figure 9).

TX Enable, SEL0=SEL1=VSS

Figure 7. TX Enable Output and Data Output

Figure 8. TX Enable Output and Data Output Zoom In