const byte serial_idle       = 0
const byte serial_send_start = 1
const byte serial_send_data  = 2
const byte serial_send_stop1 = 3
const byte serial_send_stop2 = 4
var byte serial_status       = serial_idle
var byte serial_data
var byte serial_data_count

procedure do_serial is

    -- with the clock at 10 MHz, timer0 is incremented at 2.5 MHz,
    -- which means that it overflows at 9765.625 Hz, which is close
    -- enough to 9600 baud for my liking.

    if serial_status == serial_idle then
        return
    end if

    if serial_status == serial_send_start then
        serial_pin = high
        serial_status = serial_send_data
        queue_get serial_data = q_param
        serial_data_count = 8
        return
    end if

    if serial_status == serial_send_data then
        var bit b at serial_data : 0
        serial_pin = ! b
        serial_data = serial_data >> 1
        serial_data_count = serial_data_count - 1
        if serial_data_count == 0 then
            serial_status = serial_send_stop1
        end if
        return
    end if

    if serial_status == serial_send_stop1 then
        serial_pin = low
        serial_status = serial_send_stop2
        return
    end if

    if serial_status == serial_send_stop2 then
        if queue_len > 0 then
            serial_status = serial_send_start
        else 
            serial_status = serial_idle
        end if
        return
    end if
end procedure

procedure serial_send is
    if serial_status == serial_idle then
        serial_status = serial_send_start
    end if
end procedure