USB interface for Amstrad CPC

What is it?

This is a simple to build interface, which allows to plug any USB device to the Amstrad CPC. The chip used is the WCH CH376, which handles the USB stack, but also USB Mass Storage and FAT32 file system. This makes it very, very easy to use the interface with an USB stick or hard drive and access files on the FAT32 partition directly, without using a lot of CPU power or memory on the CPC.

Wiring

The wiring is fairly simple. I used a module with a WCH376 on it for my prototype (from LC Technology). This means the interface consists only of 3 extra chips for address decoding

From the CPC bus: A15-A0, D7-D0, /IORQ, /RD, /WR are used (and of course GND and +5v)

Parts list:

• IC1: 74HCT20 - 2xQuad input NAND gate
• IC2: 74HCT02 - 4xDual input NOR gate
• IC3: 74HCT138 - 3-to-8 Demultiplexer

No schematics yet, instead here is a description of the wiring:

• Connect the power supply to IC1-3 and the WCH module. Don't forget decoupling capacitors.
• A15 to A12 go to the first NAND gate (IC1A)
• A11 to A9 and A7 go to the second NAND gate (IC1B)
• Output of both NAND gates go to the first NOR gate (IC2A).
• A8 and A6 go to the second NOR gate (IC2B)
• A5 and A4 go to the third NOR gate (IC2C)
• A2 and A1 go to the fourth NOR gate (IC2D)
• IC2B output goes to IC3.A0
• IC2C output goes to IC3.A1
• IC2D output goes to IC3.A2
• /IORQ goes to IC3.E1
• A3 goes to IC3.E2
• IC2A output goes to IC3.E3
• IC3.Y7 goes to WCH module CS
• /RD, /WR, A0, D7-D0 go directly to the WCH module
• I have not yet tried plugging INT from WCH376 to the CPC, probably it needs a diode to make it behave properly with other interrupt sources.

That was not so hard! With this wiring scheme, the WCH command/status port is &FE81. The data port is &FE80.

Note that the Y3 output of IC3 decodes FE82-FE87. This may be useful to plug another device to these addresses.

Software

The software is not written yet! But you can make sure your device is working.

10 ?hex$(inp(&FE81)) ' Status byte = 80 - Device is idle
20 out &FE81,1 : ?hex$(inp(&FE80)) ' Get chip version - 44 for mine (starts from 40)
30 out &FE81,6 : out &FE80,&55 : ?hex$(inp(&FE80)) ' Prints AA - bit-reverse of the input

The CH376 datasheets lists all the commands, and it should not be a problem to write a complete driver.