SSM2163

REV. 0

–15–

The 80C51 sends data out of its shift register LSB first, while

the SSM2163 requires data MSB first. The subroutine therefore

includes a BYTESWAP subroutine to reformat the data. This

routine transfers the MSB-first byte at location SHIFTREG to

an LSB-first byte at location SENDBYTE. The routine rotates

the MSB of the first byte into the carry with a Rotate Left

Carry instruction, then rotates the carry into the MSB of the

second byte with a Rotate Right Carry instruction. After 8

loops, SENDBYTE contains the data in the proper format.

The BYTESWAP routine in Listing 1 is convenient because the

attenuator data can be calculated in normal LSB form. For

example, fading a channel on the SSM2163 is simply a matter

of repeatedly incrementing the LEVEL_VALUE location and

calling the SEND_VAL subroutine. (Remember, the 6-bit

Listing 1. Software for the 80C51-SSM2163 Serial Port Interface

; This subroutine loads an SSM2163 mixer channel with a 6-bit

; attenuator value, and turns the Left or Right mixer switch ON or OFF.

; The attenuator value is stored at location ATTEN_VALUE

; The mixer channel address is stored at location INPUT_ADDR

;

PORT1

DATA

90H

;SFR register for port 1

ATTEN_VALUE

DATA

40H

;Attenuation level (0=0dB)

;ATTEN-VALUE: B7=0, B5–B0=Attenuation Value

INPUT_ADDR

DATA

41H

;Mixer Channel Address

;INPUT_ADDR: B7=1, B4=L chnl, B3=R chnl, B2-B0=address

LOOPCOUNT

DATA

42H

;Count loops for byte swap

SHIFTREG

DATA

43H

;Shift reg. for byte swap

SENDBYTE

DATA

44H

;Destination reg. for byte swap

;

ORG

100H

;arbitrary starting address

LD_2163:

CLR

SCON.7

;set serial

CLR

SCON.6

; data mode 0

CLR

SCON.5

;Clr SM2 for mode 0

CLR

SCON.1

;clr the transmit flag

CLR

PORT1.3

;Mute function off

SETB

PORT1.4

;WRITE and LOAD High

MOV

SHIFTREG,INPUT_ADDR

;Get mixer channel address

ACALL

SEND_IT

; send to SSM2163

SEND_VAL:

MOV

SHIFTREG,ATTEN_VALUE

;Get the attenuation level

ACALL

SEND_IT

; send it to the SSM2163

RET

;Done

;

;Convert the byte to LSB-first format and send

; it to the SSM2163

SEND_IT:

MOV

LOOPCOUNT,#8

;Shift 8 bits

BYTESWAP:

MOV

A,SHIFTREG

;Get source byte

RLC

A

;rotate MSB to carry

MOV

SHIFTREG,A

;Save new source byte

MOV

A,SENDBYTE

;get destination byte

RRC

A

;Move carry into MSB

MOV

SENDBYTE,A

;Save

DJNZ

LOOPCOUNT,BYTESWAP

;Done?

CLR

PORT1.4

;Set WRITE and LOAD low

MOV

SBUF,SENDBYTE

;Send the byte

SEND_WAIT:

JNB

SCON.1,SEND_WAIT

;Wait until 8 bits are send

CLR

SCON.1

;Clear the serial flag

SETB

PORT1.4

;Set WRITE and LOAD high to latch

RET

;

data into the SSM2163

END

The subroutine begins by setting appropriate bits in the Serial

Control register to configure the serial port for Mode 0

operation. Next the SSM2163’s Chip Select input is set low to

enable the SSM2163. The input channel address is obtained

from memory location INPUT_ADDR, adjusted to compensate

for the 80C51’s serial data format, and moved to the serial

buffer register. At this point, serial data transmission begins

automatically. When all 8 bits have been sent, the Transmit

Interrupt bit is set, and the Chip Select output is set high to

latch the channel address into the SSM2163. The subroutine

then sets Chip Select low again and proceeds to send the

attenuation value stored at location LEVEL_VALUE. When

the Chip Select input is returned high, the appropriate

SSM2163 input channel will be updated with the new

attenuation value and the subroutine ends.