A1200 Clock Port Info
Version 1.3 13th December 2003
By Ian Stedman,
On the A1200, there is an area of the motherboard that may have between 22
& 80 pins on it. This is the ubiquitous 'Clock Port.' It is a popular
interface for add on I/O card as it connects directly to part of the A1200
Why is it so Called?
According to my sources, Commodore were unsure whether they should add a
Clock to the A 1200 and how much memory to fit as standard, 1 or 2 MByte.
To allow for easily changing their minds, they built an interface
straight onto the A1200's DRAM bus and provided it with the appropriate
interface lines. Commodore decided to add a full 2 mbyte of RAM and leave
the real time clock off the A1200.
How many pins are there?
Whilst there are 80 pins, 68 of them are for the unused 1 Mbyte memory expansion and connect to the DRAM data bus. These are of no use to anyone designing an add on card.
The usable pins are located on conector P9B. P9A and P9B are next to each other on the motherboard. P9A pin 1 is marked on the PCB. If you then count along in sequence (1,3,5 top row, 2,4,6 bottom row) you will come to the first clockport pin. All numbers below are in relation to pin 1 of P9A.
19: GND Ground
20: VCC +5v DC
21: INT6 Interupt Request
23: _RTC_CS Real Time Clock CS
25: _IORD IO Read
26: _IOWR IO Write
27: A5 Address Bus Bit 5
28: A4 Address Bus Bit 4
29: A3 Address Bus Bit 3
30: A2 Address Bus Bit 2
31: D23 Data Bit Bit 23
32: D22 Data Bit Bit 22
33: D21 Data Bit Bit 21
34: D20 Data Bit Bit 20
35: D19 Data Bit Bit 19
36: D18 Data Bit Bit 18
37: D17 Data Bit Bit 17
38: D16 Data Bit Bit 16
39: GND Ground
40: _RESET Reset
Note: a _prefix e.g. _RESET denotes active low.
In summary you have an 8 bit data interface with 4 address lines and 2 chip selects and control strobes.
How do I interface to this port?
You will need to know about microprocessor interfacing to do this.
See the example interfaces in this archive for more details.
First you must be aware that there are only 4 address lines available, 2^4 gives 16 address locations per chip select. There are 2 chip selects so this yields 32 locations in total available!
Read/Write strobes are available for your use and decodes to address spaces. You can make a basic interface with minimal interface logic.
The A1200 memory map has the following addresses detailed.
$D80000 to $D8FFFF 64 KB SPARE chip select (selected by SPARE_CS)
(The area of memory where the clock port resides)
$DC0000 to $DCFFFF 64 KB Real Time Clock(RTC) (selected by RTC_CS)
The region of $DC0000 to $DCFFFF is used by an accelerator card that has a built in clock as the RTC_CS signal goes to the A1200 CPU connecotr and can not be used if you have a clock, simple really!
Address lines A2, A3, A4 and A5 only are available. You can only access D16-D23 as well so this yields the following accessible addresses,
Why are the addresses odd?
To read/write to these addresses you need to perform word read/writes under the 68000 architecture.
See the included designs for more information.
A note on chip selects and wait states
As this is a CPU bus interface
SPARE_CS 64K, 3 wait read/4 write, _IORD/_IOWR/_WAIT
Nominally an Z8530 or 1NS8250 derivative UART. Note that while slow timing
is provided there is no hardware support for the PCLK "holdoff" required
by the Z8530 chips, this must be insured by software and/or using one of
the derivative chips which minimize this requirement.
_RTC_CS 64K, 3 wait read/4 write, _IORD/_IOWR/_WAIT
Nominally a OKI M5M6242 or Ricoh RF5C01 Real-Time clock chip. Note that
there is provision for this chip both on board and on a "A501 style"
chip memory expansion header.
_NET_CS64K, 0 wait read/1 write, _IORD/_IOWR/_WAIT
Nominally a network interface chip such as the SMC C0M2020 Arcnet controller
chip or one of the various "single chip" Ethernet controller chips.
Clock Header Pinout information
Two 40 Pin DIL headers have been provided on the A1200 for a Memory/RTC expander.
This is the pinout for the complete "clock" header - the space for which is visible above and below the Chip Mem IC's. Note that only the P9B pins 19-40 actually have a header soldered in in most machines, although some earlier revisions (such as 1B) had a complete P9B, and the earliest of prototypes had both P9A and P9B complete, probably in conjunction with 1mb Chip RAM.
P9B (Bottom) | P9A (Top)
1: GND Ground | 1: GND Ground
2: VCC +5v DC | 2: VCC +5v DC
3: DRD0 DRAM Data Bus Bit 0 | 3: DRD16 DRAM Data Bus Bit 16
4: DRD15 DRAM Data Bus Bit 15 | 4: DRD31 DRAM Data Bus Bit 31
5: DRD1 DRAM Data Bus Bit 1 | 5: DRD17 DRAM Data Bus Bit 17
6: DRD14 DRAM Data Bus Bit 14 | 6: DRD30 DRAM Data Bus Bit 30
7: DRD2 DRAM Data Bus Bit 2 | 7: DRD18 DRAM Data Bus Bit 18
8: DRD13 DRAM Data Bus Bit 13 | 8: DRD29 DRAM Data Bus Bit 29
9: DRD3 DRAM Data Bus Bit 3 | 9: DRD19 DRAM Data Bus Bit 19
10: DRD12 DRAM Data Bus Bit 12 | 10: DRD28 DRAM Data Bus Bit 28
11: DRD4 DRAM Data Bus Bit 4 | 11: DRD20 DRAM Data Bus Bit 20
12: DRD11 DRAM Data Bus Bit 11 | 12: DRD27 DRAM Data Bus Bit 27
13: DRD5 DRAM Data Bus Bit 5 | 13: DRD21 DRAM Data Bus Bit 21
14: DRD10 DRAM Data Bus Bit 10 | 14: DRD26 DRAM Data Bus Bit 26
15: DRD6 DRAM Data Bus Bit 6 | 15: DRD22 DRAM Data Bus Bit 22
16: DRD9 DRAM Data Bus Bit 9 | 16: DRD25 DRAM Data Bus Bit 25
17: DRD7 DRAM Data Bus Bit 7 | 17: DRD23 DRAM Data Bus Bit 23
18: DRD8 DRAM Data Bus Bit 8 | 18: DRD24 DRAM Data Bus Bit 24
19: GND Ground | 19: GND Ground
20: VCC +5v DC | 20: VCC +5v DC
21: INT6 Interupt Request | 21: _BWE Buffered write enable
22: _SPARE_CS | 22: _ROE Buffered read enable
23: _RTC_CS Real Time Clock CS | 23: _BRAS0 Budgie Row Addr. Bit 0
24: _PWR_BAD | 24: _BRAS1 Budgie Row Addr. Bit 1
25: _IORD IO Read | 25: _BCAS_UU Budgie Col. Addr. Strobe
26: _IOWR IO Write | 26: _BCAS_UM Budgie Col. Addr. Strobe
27: A5 Address Bus Bit 5 | 27: _BCAS_LL Budgie Col. Addr. Strobe
28: A4 Address Bus Bit 4 | 28: _BCAS_LM Budgie Col. Addr. Strobe
29: A3 Address Bus Bit 3 | 29: BDRA0 Budgie DRAM Address 0
30: A2 Address Bus Bit 2 | 30: CCK_A Colour Clock
31: D23 Data Bit Bit 23 | 31: BDRA1 Budgie DRAM Address 1
32: D22 Data Bit Bit 22 | 33: BDRA8 Budgie DRAM Address 8
33: D21 Data Bit Bit 21 | 33: BDRA2 Budgie DRAM Address 2
34: D20 Data Bit Bit 20 | 34: BDRA7 Budgie DRAM Address 7
35: D19 Data Bit Bit 19 | 35: BDRA3 Budgie DRAM Address 3
36: D18 Data Bit Bit 18 | 36: BDRA6 Budgie DRAM Address 6
37: D17 Data Bit Bit 17 | 37: BDRA4 Budgie DRAM Address 4
38: D16 Data Bit Bit 16 | 38: BDRA5 Budgie DRAM Address 5
39: GND Ground | 39: GND Ground
40: _RESET Reset | 40: VCC +5v DC
NOTE: The DRAM data bus and Budgie DRAM address pins are not available for use! They connect to the 2 MByte of chip RAM fitted to the A1200.
That's it for now.
See the clk_serial.lha archive on Aminet for design notes of a clockport serial card.
Updated 14 April 2021