8253 chip

Used by standard Amstrad Serial Interface (and compatible) RS232 interfaces, mapped to Ports:

 FBDCh Amstrad RS323 8253 Baudrate Timer 0 Channel A TX Clock    (R/W)
 FBDDh Amstrad RS323 8253 Baudrate Timer 1 Channel A RX Clock    (R/W)
 FBDEh Amstrad RS323 8253 Baudrate Timer 2 Channel B RX/TX Clock (R/W)
 FBDFh Amstrad RS323 8253 Baudrate Timer 0-2 Control Registers   (W)
 Clock Input seems to be 2MHz, Clock output goes to a Z80-DART/Z80-SIO chip,
 Clock Output may be further divided by 1, 16, 32, or 64 in the DART chip

Also used by KDS Serial Interface, mapped to other Ports, and RX/TX exchanged:

 FBE8h KDS RS232 8253 Baudrate Timer 0 (RX Clock)                (R/W)
 FBE9h KDS RS232 8253 Baudrate Timer 1 (TX Clock)                (R/W)
 FBEAh KDS RS232 8253 Baudrate Timer 2 (not used)                (R/W)
 FBEBh KDS RS232 8253 Baudrate Timer 0-2 Control Registers       (W)
 Clock Input seems to be 2MHz, Clock output goes to a 6850 ACIA chip,
 Clock Output may be further divided by 1, 16, or 64 in the 6850 chip

8253 Timer 0 Register (R/W) 8253 Timer 1 Register (R/W) 8253 Timer 2 Register (R/W) These ports allow to access three decrementing 16bit timers. Assuming that the corresponding Control Registers are set to "LSB-then-MSB", the 16bit reload values (aka divider values) are written in two 8bit fractions:

 1st write: LSB of reload value
 2nd write: MSB of reload value

For the CPC RS232 interfaces, the baudrates are calculated as such:

 Baudrate  = 2MHz / reload / prescaler

Where "prescaler" is an additional divider in the DART or 6850 chip. Eg.

 300 baud  = 2MHz / 01A0h / 16
 9600 baud = 2MHz / 000Dh / 16

Note: Reading from the Timer registers does probably return the current counter values rather than the reload value (?) also not sure if/how it freezes between LSB and MSB reads, and how Control bits 4-5 are working exactly.

8253 Timer 0-2 Control Registers (W) This port allows to configure three write-only 6bit registers: Bit6-7 specify which of the registers is to be updated, Bit0-5 contain the new value for that register.

 7-6  Counter Number (0..2=Counter 0..2, 3=Reserved)
 5-4  Read/Load      (0=Latching?, 1=MSB only, 2=LSB only, 3=LSB-then-MSB)
 3-1  Mode
       0=Interrupt on Terminal Count
       1=Programmable One-Shot
       2=Rate Generator (short edge, low for 1 period of input clock)
       3=Rate Generator (square wave, low for 2nd half of counter range)
       4=Software Triggered Strobe
       5=Hardware Triggered Strobe
       6=Same as Mode 2
       7=Same as Mode 3
 0    BCD (0=normal, 1=bcd)

When used as RS232 Baudrate Generator (ie. as in the CPC), all three registers should be set to square-wave non-bcd lsb-then-msb (ie. write values 36h, 76h, and B6h to this port).