I'm reverse engineering a laser printer (Laserjet 1320) and I need to infer some things about the memory bus. The full schematic I've made while reversing is here (in pdf) on Drive. I'm not all that familiar with embedded hardware memory mapping and this part confuses me:

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The processor is a Freescale ColdFire-based device (in particular, with a ColdFire v2 core) but the particulars are proprietary. Regardless, ColdFire is basically M68k so I would assume the processor address bus is either 24 bits or full 32-bit. The program is stored in mask ROM U8, connected to the 14-bit address bus (A0:A13). Then, address bits A14:A20 are connected to D-latch U5, loaded loaded by A4:A10.

Here's the datasheet for the mask ROM, and the datasheet for the D latch. When U8 is disabled (nCE HIGH) the latch is transparent; then U8 is enabled (nCE LOW) and the latch latches. What I'm wondering is, how is this implemented on the processor side? First, a bit must be set HIGH, then bits A4:A10 are set, then the pin is set LOW, and pins A0:A13 are set. But how does the processor actually access these addresses when they're provided? Does it require two clock cycles when changing the significant bits, and if so, is this a processor issue or an external hardware issue?

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    \$\begingroup\$ It looks like a typical address bus demux scheme. If so, then in order to save pins, the processor puts out part of the address on lower-order address pins along with a control signal (that's going to the chip nCE in this case) that lets you easily drive a latch. \$\endgroup\$
    – TimWescott
    Jan 3, 2021 at 1:17

1 Answer 1


Looks like address bits A14-20 are multiplexed with A4-10. The MCU probably puts out the higher address bits first to give an external decoder time to select a memory block, then puts out the lower address bits when reading or writing to memory. This also suits the multiplexing required for DRAM, which expects row and column addresses on the same address pins as RAS and CAS are activated.

A21-23 might also be multiplexed with A11-13, which would give a 24 bit address space (16 MiB, same as the 68000) in 8 bit data bus mode. This might explain why address lines A4-A10 were chosen rather than A7-A13. In 16 bit mode A0-A22 would represent A1-A23, with upper and lower data strobes substituting for A0.


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