続続続8ピンPICでUSB －30MHz+ソフトシリアル－

PIC12F1822のRA3ポートをシリアル出力に割り当て、回路を簡素化・小型化する。

＜考え方、背景＞
前回の回路ではUSBのD+,D-,シリアルのTX,RXにI/Oピンを割り当てたため
74HC04を用いて水晶発振する回路構成でした。

余ったRA3をシリアルTXとし、D-をRA0に配置すれば、RA4-RA5に水晶を直結することが
可能になります。（シリアルTXはハードウエアからソフトウエアにスペックダウン。）



＜回路図、配線図、写真＞
この回路は従来の回路と同様に動作 します。

＜プログラム上の変更点＞

◎ CONFIG設定で_FOSC_HSを選択する(クロック入力にRA4,RA5を使用)
◎ RA0の初期設定値を出力から入力に変更
◎ D-のポートの割り当て変更により変更が必要な部分
◎ サブルーチン　ProcessOut　（以下は一例、ボーレートは9600）
※送信中に割り込みが発生すると送信不能となるため、送信中は割り込みを禁止する。

ProcessOut:
; Custom code goes here
btfss AF_BIT_INTERRUPT ; Is it an interrupt transfer?
return ; No

bcf INTCON,GIE ;割り込み禁止

; Interrupt transfer found, p
; Output Report and Send one byte.
movf RXDATA_BUFFER,W

rs1tx:
movwf rxd ;rxd=W
movlb 0x01 ;BANK1
bcf OPTION_REG,7
movlb 0x04 ;BANK4
   
bcf WPUA,TX ;スタートビット:TX=0
call bwait ;1ビット分待つ
bsf STATUS,C ;Carry=1
r1tx11:
rrf rxd,f ;Bit Output 0.13
btfss STATUS,C ; 0.13
bcf WPUA,TX ; 0.13
btfsc STATUS,C ; 0.13
bsf WPUA,TX ; 0.13
call bwait ;1ビット分待つ 0.27+102.4
bcf STATUS,C ;Carry=0 0.13
decfsz rxd,W ;8ビット送信したか? 0.13
goto r1tx11 ; 0.27 total:103.87us
bsf WPUA,TX ;ストップビット:TX=1
   
movlb 0x00 ;BANK0
bsf INTCON,GIE ;割り込み許可

bwait: ;1ビットのウエイトを作る
call bwait2 ;.27 us total:0.27+51.07+51.07=102.4 us

bwait2: ;1/2ビットのウエイトを作る
movlw .126 ;0.13 us
movwf rs ;0.13 us
decfsz rs,f ;0.13 us
goto $-1 ;0.27 us
goto $+1 ;0.27 us
return ;0.27 us total:51.07 us

＜ファームウエアのHEXファイル＞

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