8253 chip

The 8253 and 8254 are general purpose timer chips, each containing three 16bit timers.

8253 and 8254 are basically the same: both are pin-compatible and software compatible. The difference is that the 8254 supports higher clock frequencies, and it includes a readback feature for the control registers (which are write-only on 8253).

Amstrad Serial Interface

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

KDS Serial Interface

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

Aleste 520EX Serial Interface and Color Palette Swap

Also used by Aleste 520EX CPC clone:

 F4X0h Aleste PPI Port A, 8253 Baudrate Timer 0 (RX Clock)       (W)
 F4X1h Aleste PPI Port A, 8253 Baudrate Timer 1 (TX Clock)       (W)
 F4X2h Aleste PPI Port A, 8253 ColorSet Timer 2 (FUTURE)         (W)
 F4X3h Aleste PPI Port A, 8253 Timer 0-2 Control Registers       (W)

Ahhhhhhhhh, actually, above should be Port FABxh rather than F4Xxh !!! (?) Ie. first write data to F4XXh, then issue write to FABxh (with /CS and A0,A1)

 Clock Input for RX/TX is 4MHz, Clock output goes to a 8251 USART chip,
 Clock Output may be further divided by 1, 16, or 64 in the 8251 chip,
 Clock Input for FUTURE is HSYNC, Clock output selects 1st/2nd color set,
 the FUTURE clock is restarted via GATE=CRTC's "CURSOR" output,
 All registers are write-only (the 8251 chips /RD is wired to VCC)
 PPI Port A data direction must be output,
 Aleste ExtReg must enable access to 8253, and disable access to PSG

• Note: According to the Aleste schematic, clock input is 4MHZ (16MHZ/4), and it uses a russian 8253 clone. Not sure if that is correct...? Theoretically the 8253 supports only max 2.6MHZ, and anything faster requires a 8254, not a 8253.

Magic Sound Board for Aleste 520EX

The Magic Sound Board for Aleste 520EX contains two 8254 chips:

 F9D4h Magic Sound Board - 2nd 8254 - Timer 0 (DMA Channel 3)    (W)
 F9D5h Magic Sound Board - 2nd 8254 - Timer 1 (DAC Clock)        (W)
 F9D6h Magic Sound Board - 2nd 8254 - Timer 2 (not used)         (W)
 F9D7h Magic Sound Board - 2nd 8254 - Timer 0-2 Control          (W)
 F9D8h Magic Sound Board - 1st 8254 - Timer 0 (DMA Channel 0)    (W)
 F9D9h Magic Sound Board - 1st 8254 - Timer 1 (DMA Channel 1)    (W)
 F9DAh Magic Sound Board - 1st 8254 - Timer 2 (DMA Channel 2)    (W)
 F9DBh Magic Sound Board - 1st 8254 - Timer 0-2 Control          (W)
 Clock source for all six timers is (inverted) 4MHz from expansion port.

• 8253/8254 Timer 0 Register (R/W)
• 8253/8254 Timer 1 Register (R/W)
• 8253/8254 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.

• 8253/8254 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, and 8253: 3=Reserved, or 8254: 3=Readback)
 5-4  Read/Load      (0=Latching?, 1=LSB only, 2=MSB 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).

• Media:8253.pdf - 8253 Datasheet (slower clock, write-only control registers)
• Media:8254.pdf - 8254 Datasheet (faster clock, read/write-able control registers)