AN1833 Freescale Semiconductor / Motorola, AN1833 Datasheet - Page 23

• Manufacturer Part Number: AN1833
• Description: A Low-Power Wireless Remote Control Transmitter and Receiver
• Manufacturer: Freescale Semiconductor / Motorola

Sending
a Message
Acquire and
Validate an
Access Request
Permit or Reject
Access
AN1833
MOTOROLA
When two keypad values have been received, the MCU ensures a logic
low is on the TTL (transistor-to-transistor logic) data input to the
transmitter and enables the MC13176 via output pins on port B. After
waiting for the transmitter to warm up, a logic 1 is presented to the TTL
data inputs of the MC13176 for a 500-microsecond interval representing
a “front porch." Three preamble characters (55 hex) and the two keypad
values received are sent serially to the transmitter by the
MC68HC705J1A using the routines written by Scott George [13]. The
MCU turns off the transmitter enable pin on port B, initializes all software
flags and buffers, prepares for the next keypad interrupt, and then re-
enters sleep mode. The transmitter flowchart is shown in
Figure
The receiver activates itself when it sees a specific signal exceed a
strength threshold and passes the data bits received in a digital format
to an MCU input port. When the MCU sees at least a 500-microsecond
logic 1 on the receiver input pin, a “shutter timer” is activated that has a
timing interval equal to 8.2 milliseconds.
The MC68HC705J1A examines the received data, using the routines
written by Scott George [13], for ASCII character sequences of:
The character data is put together via several subroutines and placed in
the MCU buffer memory. Data acquisition is terminated when an
interrupt caused by the “shutter timer” interval expiring occurs. The
receiver flowchart is shown in
The receiver buffer is examined for a specific data sequence when data
acquisition is terminated. If at least two of the preamble characters are
received as alternating data bits of logic 1 and 0 (ASCII 55 hex) and the
receiver number or a broadcast number is in the lower nibble of the
address byte, then the MCU will perform actions specified in the
command byte.
•
•
•
Freescale Semiconductor, Inc.
For More Information On This Product,
Start bit
Eight bits of logic high or low data, and
A stop bit during the “shutter timer” period
10.
Go to: www.freescale.com
Microcontroller Software Overview: Initiate an Access Request
Figure
11,
Figure
12, and
Figure 9
Figure
Application Note
13.
and
23