On Tue, 27 May 2014, Michael Schmitz wrote:
so you are saying the head.S setup uses PCLK as clock source in x16
mode, while the serial driver does something more complicated to use the
baud rate generator with a lower frequency clock?
It seems so. The baud rate table in Atari_SCC.c defines both a clock
source and time constant (divisor) for each speed.
static BAUD_ENTRY bdtab_norm[20] = {
/* B0 */ { 0, 0 },
/* B50 */ { CLK_RTxC, 4590 },
/* B75 */ { CLK_RTxC, 3060 },
/* B110 */ { CLK_PCLK, 4576 },
/* B134 */ { CLK_PCLK, 3756 },
/* B150 */ { CLK_RTxC, 1530 },
/* B200 */ { CLK_PCLK, 2516 },
/* B300 */ { CLK_PCLK, 1678 },
/* B600 */ { CLK_PCLK, 838 },
/* B1200 */ { CLK_PCLK, 420 },
/* B1800 */ { CLK_PCLK, 280 },
/* B2400 */ { CLK_PCLK, 210 },
/* B4800 */ { CLK_RTxC, 48 },
/* B9600 */ { CLK_RTxC, 24 },
/* B19200 */ { CLK_RTxC, 12 },
/* B38400 */ { CLK_RTxC, 6 }, /* #15 spd_extra */
/* B57600 */ { CLK_RTxC, 4 }, /* #16 spd_hi */
/* B115200 */ { CLK_RTxC, 2 }, /* #17 spd_vhi */
/* B230400 */ { CLK_RTxC, 1 }, /* #18 spd_shi */
/* B460800 */ { 0, 0 } /* #19 spd_warp: Impossible */
};
The x16 mode is set by SCC_change_speed() when time constant == 1 or time
constant > 4 though I don't understand why that is.
Atari_SCC.c, for 38400 baud, in SCC_change_speed() essentially does,
AUX1_CTRL_REG = 0x44
CLK_CTRL_REG = 0x50
DPLL_CTRL_REG = 0x00
TIMER_LOW_REG = 1
TIMER_HIGH_REG = 0
DPLL_CTRL_REG = 0x01
That is, set the BRG clock source to RTxC pin and time constant to 1,
which would give 38250 baud for a 3.6720 MHz clock source.
Whereas, head.S for 9600 baud does something like this,
AUX1_CTRL_REG = 0x44
CLK_CTRL_REG = 0x50
TIMER_LOW_REG = 24
TIMER_HIGH_REG = 0
DPLL_CTRL_REG = 0x02
DPLL_CTRL_REG = 0x03
That is, set the BRG clock source to PCLK and time constant to 24, which
would give 9680.2 baud for a 8.0539 MHz clock source.
Problem: bdtab_norm[] above specifies { CLK_RTxC, 24 } for 9600 baud. This
doesn't make sense. Perhaps the logic in SCC_ioctl() in Atari_SCC.c fixes
it up for speeds specified by the user? Who knows.
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