L1B MK2A Operation and Construction Notes
This is a respin of the MK2 board adding the following features
- Clock filter on the incoming Phi0 from the BBC (with optional bypass through jumper pins)
- Pull-up resistor on the CPU RDY signal
- Replacement of the 74HCT373 Octal latch with an XC9536 CPLD
- Deletion of the power connection
- Addition of a block of power and GPIO pins to be used as jumper options or probe points
- Dedicated probe point for the main CPLD input clock
Beeb816 has one dedicated internal register which is nominally mapped to 0x800000, but decoded lazily to match only the top two bits of the bank address.
| Bit | Name | Reset Val | Function |
|---|---|---|---|
| 0 | Clock Divider Low | 0 | Divider bits select divide by 1 (00), 2(01), 4(10 or 11) |
| 1 | Clock Divider High | 0 | |
| 2 | EnableHSClk | 0 | when set, enables high speed clock operation |
| 3 | VRAM size | 0 | set size of VRAM to 31K (0) or 20K (1) |
| 4 | MAP_ROM | 0 | Remaps ROM 4-7, 12-15 accesses to fast RAM |
| 5 | HostId[0] | 0 | Identifies host machine as BBC B (00), BBC B+ (01), Electron (10) or Master (11) |
| 6 | HostId[1] | 0 | |
| 7 | Shadow RAM | 0 | Select between Shadow RAM and cached VRAM from 0x3000-0x7FFF |
Currently the HostID bits are read only and capture the state of jumpers on the j[0:1] pins. The intention is for the Boot ROM to identify the host and program these appropriately on start up eventually, making the jumpers redundant.
Bit 3 selects the amount of memory which Beeb816 considers to be a write-through cache for the video display. On startup this is set to 31K for best compatibility with games and demos which might move the video area around in the lower memory. By setting the VRAM size bit to 1 this can be reduced the the 20K area immediately below &8000 - ie the default area where the display would be for BBC BASIC. The smaller VRAM size gives a speed advantage because more of the machines RAM can be treated as fast for both reads and writes; all writes to the VRAM area have to slow down to the motherboard's 2MHz clock.
The following table is an example BOM from Digi-Key.
| Manufacturer Part Number | Manufacturer | Digi-Key Part Number | Customer Reference | Reference Designator | Quantity | Unit Price | Extended Price | Description |
|---|---|---|---|---|---|---|---|---|
| AS6C4008-55PCN | "Alliance Memory, Inc." | 1450-1027-ND | 512Kx8 SRAM | SRAM | 1 | 3.59000 | £3.59 | IC SRAM 4M PARALLEL 32DIP |
| A 32-LC-TT | Assmann WSW Components | AE10354-ND | 32pin DIL Socket | SRAM (Socket) | 1 | 0.36000 | £0.36 | CONN IC DIP SOCKET 32POS TIN |
| EEA-GA1E220H | Panasonic Electronic Components | P15803CT-ND | 22uF Electrolytic Cap | C22U_0 | 1 | 0.18000 | £0.18 | CAP ALUM 22UF 20% 25V RADIAL |
| PRPC040SAAN-RC | Sullins Connector Solutions | S1011EC-40-ND | Pin strip for JTAG/GPIO pins | JTAG, GPIO | 1 | 0.51000 | £0.51 | CONN HEADER VERT 40POS 2.54MM |
| A-CCS 084-Z-T | Assmann WSW Components | AE11110-ND | PLCC84 Socket | CPLD (Socket) | 1 | 1.20000 | £1.20 | IC PLCC SOCKET 84POS TIN |
| A 40-LC-TT | Assmann WSW Components | AE10008-ND | 40 pin DIP Socket for CPU | CPU (Socket) | 1 | 0.39000 | £0.39 | CONN IC DIP SOCKET 40POS TIN |
| XG4A-4034 | Omron Electronics Inc-EMC Div | OR893-ND | 40 pin PCB header (latching) | CON | 1 | 1.93000 | £1.93 | CONN HEADER R/A 40POS 2.54MM |
| ADIP 40Z-LC | Assmann WSW Components | HDP40S-ND | 40 Pin DIL Header | None | 1 | 1.56000 | £1.56 | CONN DIP HDR IDC 40POS VERT |
| 1658621-9 | TE Connectivity AMP Connectors | AKC40H-ND | 40 Pin IDC Female Connector | None | 1 | 1.54000 | £1.54 | CONN RCPT 40P IDC 26-28AWG GOLD |
| 3365/40 300 | 3M | 3M157807-1-ND | Ribbon Cable | None | 1 | 1.47000 | £1.47 | CBL RIBN 40COND 0.050 GRAY 1' |
| ACO-50.000MHZ-EK | Abracon LLC | 535-9219-5-ND | TTL Oscillator | OSC | 1 | 1.45000 | £1.45 | XTAL OSC XO 50.0000MHZ HCMOS TTL |
| 1107741 | Aries Electronics | A462-ND | Oscillator Socket | OSC (Socket) | 1 | 0.58000 | £0.58 | OSCILLATOR SOCKET FULL SIZE 4PIN |
| B32529C1104J289 | EPCOS - TDK Electronics | 495-2479-1-ND | Decoupling Caps | C100N_1-C100N_6 | 6 | 0.25000 | £1.50 | CAP FILM 0.1UF 5% 100VDC RADIAL |
| 940-44-044-24-000000 | Mill-Max Manufacturing Corp. | ED90008-ND | CPLD2 Socket | CPLD2 | 1 | 1.36000 | £1.36 | CONN SOCKET PLCC 44POS TIN |
| FCA0805C104M-J2 | Cornell Dubilier Electronics (CDE) | 338-2772-1-ND | SMD decaps | cap* | 3 | 0.56000 | £1.68 | CAP FILM 0.1UF 20% 16VDC 0805 |
| RNF14FTD100R | Stackpole Electronics Inc | RNF14FTD100RCT-ND | Clock Filter and Databus | R100_* | 9 | 0.08000 | £0.72 | RES 100 OHM 1/4W 1% AXIAL |
| RNMF14FAD47K0 | Stackpole Electronics Inc | RNMF14FAD47K0CT-ND | RDY Pullup | R47K | 1 | 0.08000 | £0.08 | RES 47K OHM 1/4W 1% AXIAL |
| SR151A470KAR | AVX Corporation | 478-11006-ND | Clock filter Cap | C47PF | 1 | 0.23000 | £0.23 | CAP CER 47PF 100V C0G/NP0 RADIAL |
| PRPC040SAAN-RC | Sullins Connector Solutions | S1011EC-40-ND | PCB Pins | 1 | 0.51000 | £0.51 | CONN HEADER VERT 40POS 2.54MM | |
| QPC02SXGN-RC | Sullins Connector Solutions | S9337-ND | Jumper Header | 1 | 0.08000 | £0.08 | "CONN JUMPER SHORTING .100, GOLD" | |
| ACO-12.000MHZ-EK | Abracon LLC | 535-9205-5-ND | 12MHz Oscillator | OSC | 1 | 1.45000 | £1.45 | XTAL OSC XO 12.0000MHZ HCMOS TTL |
Additionally you will need to source the following parts
- WDC 65C816 CPU in a 40 Pin DIL package
- Xilinx XC95108 CPLD in a PLCC84 package
- Xilinx XC9536 CPLD in a PLCC36 package
- Beeb816 Mk2A PCB
