As part of my BBC Micro PC project (more about this later) I 117 | need to turn the keyboard in a BBC Micro in to a USB hid device. I could 118 | just rip the insides out of a PC USB keyboard and solder wires on to 119 | the back off all the key switches but that seems somewhat messy. So I 120 | decided to design a micro-controller circuit to read the keyboard and to 121 | appear as a standard USB HID device. The neatest solution is to have 122 | the micro-controller drivng the keyboard circuitry the same was as the 123 | BBC Micro main board does. This post explains how the keyboard circuit 124 | works.The BBC Model B keyboard circuit board 125 | is more than just a grid of key-switches. It comprises some circuitry to 126 | reduce the number of connections between it and the main circuit board 127 | to 8 lines and to generate interrupts on key presses. Its a very neat 128 | design. Below is the keyboard circuit diagram taken from the BBC Model B 129 | service manual. The diagram also includes a bit of circuitry for the 130 | speech ROM expansion which I will ignore.
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| IC1 | 136 |74xx161/163 | 137 |Synchronous 4 bit binary counter | 138 |
|---|---|---|
| IC2 | 141 |74xx45 | 142 |BCD to decimal decoder | 143 |
| IC3 | 146 |74xx251 | 147 |3 state 1 of 8 line data selector | 148 |
| IC4 | 151 |74xx30 | 152 |Octal NAND gate | 153 |
At the core is the traditional key matrix. It is a grid of rows and 157 | columns with momentary push switches that connect a row to a column when 158 | pressed. All of the rows are tied high by resistors to the 5v supply 159 | rail. The rows are inputs to an octal NAND gate (IC4), which, in this 160 | case is better thought of as an OR gate with inverting inputs. I.e. but 161 | default the output of the NAND gate is low but if one of the rows is 162 | pulled low the NAND output goes high. The rows are also inputs in to the 163 | line data selector (IC2).
164 |The synchronous 4 bit binary counter (IC1) is driven by the BBC 165 | Micro’s 1MHz clock and when the KB EN is high is in free running mode. 166 | The 4 bit outputs (outputs QA-QD) are decoded by the BCD to decimal 167 | decoder (IC3) such that the decoder’s outputs go low in turn. The 168 | outputs of the decoder form the columns of the grid. I.e. column 0 is 169 | low on the first clock pulse, column 1 on the second and so on. If a key 170 | is pressed the row the key is on will go low when the column goes low. 171 | This in turn makes the output of the NAND gate go high indicating a key 172 | is pressed. The output of the NAND gate is used to generate an 173 | interrupt in the BBC Micro’s CPU.
174 |So normally the keyboard is being scanned by the hardware without any 175 | software influence. This is a good thing as it takes a lot of loading 176 | off of the BBC Micro’s CPU. When the key press interrupt is triggered 177 | the key scanning software takes over. As I said normally the 4 bit 178 | binary counter (IC1) is free running however if the LOAD pin (pin 9) is 179 | taken low the 4 bit input value (inputs A-D) are latched through to the 180 | outputs (QA-QD) and hence control the columns provided by the decoder 181 | (IC3). IC1’s load pin is connected to KB EN.
182 |The S line of the data line selector (IC2) is the select line and is 183 | also connected to KB EN. The output of IC2 is 3 state meaning it either 184 | reflects the appropriate data input line when select is low or is high 185 | impedance. The data input lines are the row lines of the keyboard 186 | matrix. The input line is chosen using the three bit ‘address’ value on 187 | A-C. The row state is output via pin W.
188 |So the keyboard is normally hardware scanning. If the user presses a 189 | key, the interrupt is triggered. The CPU enables the keyboard by setting 190 | KB EN low and then manually scans the key board by setting the columns 191 | in turn via IC1 and checking the state of the rows in turn via IC2. 192 | Once the scan is complete KB EN is set high and the hardware scanning 193 | continues.
194 |There are three keys on the keyboard which are exceptions. The two 195 | SHIFT and CTRL keys are on a row that does not trigger an interrupt. 196 | This means that shift is only scanned manually or when another key is 197 | pressed. The BREAK key (labled RST) is completely separate from the 198 | matrix and triggers the CPU’s reset line. On boot the CPU scans the 199 | keyboard manually to see if any of the modifiers are held down to do a 200 | cold reset or mount the disk for example.
201 |The BBC Micro keyboard can be easily interfaced with say an ATMEL 202 | ATMega328 (like in an Arduino). The micro-controller can drive a pin 203 | directly from one of the timers generating a square wave output to scan 204 | the keyboard. The interrupt can be connected to one of interrupt inputs 205 | and the rest connected to general purpose IO pins set as outputs or 206 | inputs as appropriate. Dick Streefland’s USBtiny 207 | code provides the functionality to support the USB protocol in software 208 | utilising three IO pins on the micro-controller and a few discrete 209 | components. All that is required by me is to write the keyboard scanning 210 | code and provide the missing bits of the HID device profile.
211 |
Very nice description.
245 |What are you up to? 😉
246 |I’ve got an idea for a similar sounding project using a Beagleboard 247 | running RiscOS. Saw it at the Acorn show at Wakefield last year. Now the 248 | Beagle is available again (it was in the Wayback machine at MakerFaire) 249 | I might just pick up this project again.
250 |B
251 |Reply
252 |Today I was looking at the BBC keyboard I have with a view of 265 | adapting it for use on the ZX81. I came across this page and wondered if 266 | you’d seen it?
268 |267 | http://denki.world3.net/beebkb.html
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269 |Hi – keen to know if you got this working: As it happens I had 282 | arrived at the same conclusion and bought an Arduino Leonardo for the 283 | job (as it has the built-in USB keyboard mode support). I am happy to 284 | share my progress as I get going on this.
285 |regards
287 |286 | Paul
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288 |