68HC05K0 查看數據表（PDF） - Motorola => Freescale

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68HC05K0

Infra-red Remote Control

Motorola => Freescale 

Software

The listing of the remote control assembler code is

contained at the end of this application note. The

first section of the listing sets up the ports prior to

going into STOP mode and waiting for a key to be

pressed. PortA bits 0-3 are set up as inputs with the

pull-downs enabled. Bits 4-7 are set up as outputs

logic ‘1’ as is PortB bit 0. PortB bit 1 is set-up as

output logic ‘0’ to switch off the IR amplifier before

going into STOP mode.

The next section of code named ‘presd’ is the

routine pointed to by the interrupt vector and is

entered when a key is pressed. This routine first

calls the keyboard scanning routine to determine

which key has been pressed. It then calls the

decoding routine to convert the code from the

keyboard to a code that will be accepted by the

television. The start message is then transmitted

and is followed by the instruction message. There

is then a check to see if the same key is still being

pressed. If it is then the instruction message is

re-transmitted until the key is released and the end

message is transmitted.

As the transmission protocol requires nine data bits

and only one byte instructions are being decoded a

flag has to be set for the ninth bit of the

transmission routine. For the start and end

transmissions this flag is set to 1 to give the nine

1’s message. For all instructions the ninth bit is 0 so

the flag is cleared.

The decoding routine compares the code from the

keyboard scan routine with data array ‘keydat’. On

a match it takes the corresponding element from

the array ‘tvdat’ as the instruction code for

transmission.

The values of the instruction codes shown in the

right-hand side of Figure 1 are specific for the

receiver application. Each receiver using the same

communications protocol will receive the same

nine bit instruction but what the instruction does is

dependent upon the receiver software. In this

example the eight bit instruction ‘14’ changes the

channel to number four. In another receiver

application the receiver may interpret the

instruction code ‘14’ as increase volume.

The transmission routine is entered with the

instruction for transmission in ‘keyst3’. After the

pre-bit and the start-bit are transmitted the

instruction byte is rotated (LSB first) into the carry

flag. A logic ‘1’ is sent for transmission if the flag is

set after rotation and a logic ‘0’ is sent for

transmission if the flag is cleared. Each bit is

transmitted as shown in Figure 1. The routines

‘send0’ and ‘send1’ send a pause of 512µs

followed by a 32kHz pulse train for 512µs and a

32kHz pulse train for 512µs followed by a 512µs

pause respectively. In the situation when a ‘1’

follows a ‘0’ then a pulse train of 1024µs is

required. To avoid breaks in this pulse train the

‘send0’ routine checks the next bit to be

transmitted to see if a double length pulse train

must be transmitted. The ‘send1’ routine then has

to check that a double length pulse train has not

been sent in the previous one and a half bit periods

before sending a pulse train.

The routine ‘burst’ produces the 32kHz pulse train for

a duration set by a count in the accumulator. As the

instruction time for setting the PortB bit 1 pin high or

low is five clock cycles then the minimum processor

clock period is derived by dividing the minimum

output state time, which is 8µs when the output is

high, by the minimum number of clock cycles to

change this state. This gives an internal clock period

of 8µs/5 equalling 1.6µs. Adding a three cycle delay

will require an internal clock period of 8µs/8 = 1µs,

allowing a 2MHz oscillator to be used.

The code size is approximately 300 bytes, leaving

memory space for more features to be added to the

controller.

MOTOROLA

6

AN463/D

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