gc_n64_usb-v3/gcn64txrx.S

250 lines
4.4 KiB
ArmAsm
Raw Normal View History

#include <avr/io.h>
.text
.global gcn64_sendBytes
.global gcn64_receiveBits
#define xl r26
#define xh r27
#define yl r28
#define yh r29
#define zl r30
#define zh r31
#define __zero_reg__ r1
/* the value of the gpio is pre-configured to low. We simulate
an open drain output by toggling the direction */
#ifdef STK525
#define GCN64_DATA_PORT _SFR_IO_ADDR(PORTA)
#define GCN64_DATA_DDR _SFR_IO_ADDR(DDRA)
#define GCN64_DATA_PIN _SFR_IO_ADDR(PINA)
#else
#define GCN64_DATA_PORT _SFR_IO_ADDR(PORTD)
#define GCN64_DATA_DDR _SFR_IO_ADDR(DDRD)
#define GCN64_DATA_PIN _SFR_IO_ADDR(PIND)
#endif
#define GCN64_DATA_BIT 0
#if F_CPU != 16000000L
#error Only 16MHz clock supported
#endif
/************************************************
* Function gcn64_receiveBits
*
* Works by timing the low and high periods of each bit and
* then comparing them to know if a 1 or a 0 was on the wire.
*
* Example timed values:
* 54 4e 53 4d 53 4d 53 4e 53 4d 4d 53 53 4e 4c 54 53 4d 54 4d 53 4d 53 4d ...
* 0 0 0 0 0 1 0 1 0 0 0 0 ....
*/
; The bit timeout is a counter to 127. This is the
; start value. Counting from 0 takes hundreads of
; microseconds. Because of this, the reception function
; "hangs in there" much longer than necessary..
#define TIMING_OFFSET 75
; unsigned int gcn64_receiveBytes(unsigned char *dstbuf, unsigned char max_bytes);
; r24,r25 : dstbuf
; r22 : max bytes (for fututre use)
; return: count in r24,r25
gcn64_receiveBits:
clr xl
clr xh
mov zl, r24
mov zh, r25
clr r18
clr r20
inc r20
initial_wait_low:
inc r18
breq timeout ; overflow to 0
sbic GCN64_DATA_PIN, GCN64_DATA_BIT
rjmp initial_wait_low
; the next transition is to a high bit
rjmp waithigh
waitlow:
mov r21, r18 ; Save the last value
ldi r18, TIMING_OFFSET
waitlow_lp:
inc r18
brmi timeout_waitlow ; > 127 (approx 50uS timeout)
sbic GCN64_DATA_PIN, GCN64_DATA_BIT
rjmp waitlow_lp
adiw xl, 1 ; count this bit
; TODO : Check maximum size
;breq overflow ; > 255
#if 0
st z+, r19
st z+, r18
rjmp waithigh
#else
; Compare the low period and the high period.
sub r19, r18
brcs got1
got0:
st z+, __zero_reg__
rjmp waithigh
got1:
st z+,r20
#endif
waithigh:
ldi r19, TIMING_OFFSET
waithigh_lp:
inc r19
brmi timeout ; > 127
sbis GCN64_DATA_PIN, GCN64_DATA_BIT
rjmp waithigh_lp
rjmp waitlow
timeout_waitlow:
adiw xl, 1 ;
; TODO : Check maximum size
;breq overflow ; > 255
; Compare the low period and the high period.
sub r19, r21
brcs lastwas1
lastwas0:
st z+, __zero_reg__
rjmp rxdone
lastwas1:
st z+,r20
rjmp rxdone
overflow:
clr xl
clr xh
timeout:
rxdone:
; Return the number if received bits in r24,r25
mov r24, xl ; yl
mov r25, xh ; yh
ret
/************************************************
* Send data using the N64/GC serial protocol which
* is as follows:
* 0 1
* __ _____
* ____| __|
* ^ ^ ^ ^ ^ ^
* 3us 1us 1us 3us
*
* To send a 1, the pin direction is set to input.
* To send a 0, the pin direction is set to output.
* (of course, it's value is preset to zero)
*
* At 16 mhz, a 1us period is 16 cycles. Thus a 3us period
* is 48 cycles.
*
* Pointer to data is passed in r24, r25
* Number of bytes to send is passed in r22
*
* A stop bit is added at thy end of the packet.
*
************************************************/
gcn64_sendBytes:
; Move r23,r24 pointer to z
mov zl, r24
mov zh, r25
tst r22
breq done_send
send_next_byte:
; Check if this is the last byte.
tst r22
breq send_stop
dec r22
ld r21, z+
ldi r27, 0x80 ; mask
send_next_bit:
mov r19, r21
and r19, r27
brne send1
nop
send0:
sbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Pull bus to 0
ldi r20, 15
lp_send0_3us:
dec r20
brne lp_send0_3us
nop
cbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Release bus to 1
lsr r27
breq send_next_byte
nop
nop
nop
nop
nop
nop
rjmp send_next_bit
send1:
sbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Pull bus to 0
ldi r20, 4
lp_send1_1us:
dec r20
brne lp_send1_1us
nop
nop
cbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Release bus to 1
ldi r20, 10
lp_send1_3us:
dec r20
brne lp_send1_3us
nop
nop
lsr r27
breq send_next_byte
nop
nop
nop
nop
nop
nop
rjmp send_next_bit
send_stop:
nop
nop
nop
nop
nop
nop
nop
nop
; STOP BIT
sbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Pull low for stop bit
ldi r20, 4
stbdly0:
dec r20
brne stbdly0
nop
cbi GCN64_DATA_DDR, GCN64_DATA_BIT ; Release
done_send:
ret