Multidrop Mode differentiates the data characters and the address
characters. Data is transmitted with the parity bit to 0 and addresses
are transmitted with the parity bit to 1.
Usually multidrop is implemented by forcing MARK and SPARE parity as follow:
/* Write data */
/* Address byte */
term.c_cflag |= PARENB | CMSPAR | PARODD; /* MARK(1) */
ret = tcsetattr(fd, TCSADRAIN, &term);
if (ret < 0) {
perror("unable to set MARK");
exit(-1);
}
buf[0] = 0x00;
ret = writen(fd, buf, 1);
if (ret < 0) {
perror("unable to write address (%m)");
exit(-1);
}
/* Data bytes */
term.c_cflag |= PARENB | CMSPAR; /* SPACE(0) */
term.c_cflag &= ~PARODD;
ret = tcsetattr(fd, TCSADRAIN, &term);
if (ret < 0) {
perror("unable to set SPACE");
exit(-1);
}
buf[0] = 0x1c;
buf[1] = 0x01;
buf[2] = 0x01;
buf[3] = 0x00;
buf[4] = 0x01;
buf[5] = 0xde;
buf[6] = 0xd4;
ret = writen(fd, buf, 7);
if (ret < 0) {
perror("unable to write data (%m)");
exit(-1);
}
However this usually slow down communication by adding a delay between
the first byte and the others.
The first attached patch defines two non-stadard bits PARMD (that
enables multidrop) and SENDA (that marks the next transmitted byte as
address) that can be used to completely remove the delay by managing
the correct parity bit generation in hardware (the second patch is for
SAMA5D3 CPU). Then the userland code becomes:
/* Transmission: enable parity multidrop and mark 1st byte as address */
term.c_cflag |= PARENB | CMSPAR | PARMD | SENDA;
ret = tcsetattr(fd, TCSADRAIN, &term);
if (ret < 0) {
perror("unable to set MARK");
exit(-1);
}
/* Write data */
n = 0;
buf[n++] = 0x00;
buf[n++] = 0x1c;
buf[n++] = 0x01;
buf[n++] = 0x01;
buf[n++] = 0x00;
buf[n++] = 0x01;
buf[n++] = 0xde;
buf[n++] = 0xd4;
ret = writen(fd, buf, n);
if (ret < 0) {
perror("unable to write data (%m)");
exit(-1);
}
Rodolfo
Rodolfo Giometti (2):
tty: add bits to manage multidrop mode
tty serial: add multidrop support for atmel serial controllers
drivers/tty/serial/atmel_serial.c | 14 ++++++++++----
include/linux/tty.h | 2 ++
include/uapi/asm-generic/termbits.h | 2 ++
3 files changed, 14 insertions(+), 4 deletions(-)
--
2.7.4
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