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/*
* Copyright © 2012 Keith Packard <keithp@keithp.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; version 2 of the License.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
*/
#include <ao.h>
#include <ao_async.h>
#define AO_ASYNC_BAUD 38400l
#define AO_ASYNC_DELAY (uint8_t) (1000000l / AO_ASYNC_BAUD)
#define LED_PORT PORTB
void
ao_async_start(void)
{
LED_PORT |= (1 << AO_LED_SERIAL);
}
void
ao_async_stop(void)
{
LED_PORT &= ~(1 << AO_LED_SERIAL);
}
void
ao_async_byte(uint8_t byte)
{
uint8_t b;
uint16_t w;
uint8_t v;
uint8_t bit;
uint8_t w_hi, w_lo;
/* start data stop */
w = (0x000 << 0) | (byte << 1) | (0x001 << 9);
w_hi = w >> 8;
w_lo = w;
ao_arch_block_interrupts();
#if AO_LED_SERIAL != 4
#error "expect AO_LED_SERIAL to be 4"
#endif
/* Ok, this is a bit painful.
* We need this loop to be precisely timed, which
* means knowing exactly how many instructions will
* be executed for each bit. It's easy to do that by
* compiling the C code and looking at the output,
* but we need this code to work even if the compiler
* changes. So, just hand-code the whole thing
*/
asm volatile (
" ldi %[b], 10\n" // loop value
"loop:\n"
" in %[v], %[port]\n" // read current value
" andi %[v], %[led_mask]\n" // mask to clear LED bit
" mov %[bit], %[w_lo]\n" // get current data byte
" andi %[bit], 0x01\n" // get current data bit
" swap %[bit]\n" // rotate by 4 (AO_LED_SERIAL is 4)
" andi %[bit], 0xf0\n" // mask off other 4 bits
" or %[v], %[bit]\n" // add to register
" out %[port], %[v]\n" // write current value
" lsr %[w_hi]\n" // shift data
" ror %[w_lo]\n" // ...
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" nop\n"
" subi %[b], 1\n" // decrement bit count
" brne loop\n" // jump back to top
: [v] "=&r" (v),
[bit] "=&r" (bit),
[b] "=&r" (b),
[w_lo] "+r" (w_lo),
[w_hi] "+r" (w_hi)
: [port] "I" (_SFR_IO_ADDR(LED_PORT)),
[led_mask] "M" ((~(1 << AO_LED_SERIAL)) & 0xff)
);
#if 0
/*
* Here's the equivalent C code to document
* what the above assembly code does
*/
for (b = 0; b < 10; b++) {
uint8_t v = LED_PORT & ~(1 << AO_LED_SERIAL);
v |= (w & 1) << AO_LED_SERIAL;
LED_PORT = v;
w >>= 1;
/* Carefully timed to hit around 9600 baud */
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
asm volatile ("nop");
}
#endif
ao_arch_release_interrupts();
}
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