2022-11-25 03:57:01 +00:00
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---
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2023-09-17 23:29:21 +00:00
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title: Printed Circuit Boards
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permalink: /docs/pcb
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exerpt: "Printed Circuit Boards"
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2022-11-25 03:57:01 +00:00
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---
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Although it was originally designed as a one-off programmer to be used on a breadboard,
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there are now some PCB versions.
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## 28C256 PCB
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A PCB version for the 28C256 and 28C64 chips was built to support a SAP-1 TTL breadboard
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computer. This project required a lot of burns for its microcode and the PCB version
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with the ZIF socket was much quicker than prying the chips out of a breadboard without
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disturbing any of the wires.
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The [KiCad design files](https://github.com/TomNisbet/TommyPROM/tree/master/schematics)
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are in the project repo.
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2023-09-17 23:29:21 +00:00
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[](images/TommyPROM-pcb.jpg)
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2022-12-12 23:58:45 +00:00
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## TommyPROM32
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TommyPROM32 was built to support SST39SF0x0 and other 32-pin EEPROM and Flash chips. While
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providing a stable platform for the SST39SF chips, the board can serve as a test bed for
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other chips as well. It uses a set of headers that allow any of the signals to the target
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chip to be re-routed. The default configuration, with just jumper shunts installed, is
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wired for the SST39SF0x0 chips.
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2023-09-17 23:29:21 +00:00
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[](images/TommyPROM32-SST39SF.jpg)
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Many other chips, particularly if they follow JEDEC standards, can be supported by
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removing shunts and adding just a few jumper wires.
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Chips with fewer pins can also be supported. There are power pins in place to support
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28-pin and 24-pin chips with a shunt to the correct VCC pin. The photo below shows the
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28C256 with a power jumper on pin 28, the _WE_ signal moved down to pin 27, and the _A14_
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signal moved from ping 27 to pin 1.
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2023-09-17 23:29:21 +00:00
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[](images/TommyPROM32-28C256.jpg)
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Power inputs and a switch are provided to support older flash and EEPROM chips that need
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higher programming voltages. Many of these chips just need a higher voltage, like 12V,
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applied to the VPP pin continuously during the programing and verification cycle. In
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normal operation, the 5V VCC signal is applied to the VPP pin. The board has VPP inputs
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for two voltages. Applying 12V to _VPP In High_ and 5V to _VPP In Low_ lets the VPP
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output be switched between these two voltages using a manual slide switch for programming.
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The photo below shows the WE27C257 set up for programming. The _VPP in Low_ signal is
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connected to _VCC_ and the _VPP in High_ signal is using 12V from an external power
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supply. The _VPP Out_ signal is connected to the chip's VPP on pin 1. The chip is in
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programming mode when the slide switch is set to the _High_ position and is in read mode
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when in the _Low_ position.
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2023-09-17 23:29:21 +00:00
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[](images/TommyPROM32-27C257-pgm.jpg)
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To erase the WE27C257 chip, the external power supply is providing 14V and the _VPP Out_
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is connected to the chip's _VPP_ and _A9_ pins. An external breadboard was needed to
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allow the _VPP Out_ signal to be connected to multiple pins. A new spin of the board is
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planned with multiple _VPP Output_ pins and some general-purpose interconnect pins.
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2023-09-17 23:29:21 +00:00
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[](images/TommyPROM32-27C257-erase.jpg)
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2022-11-25 13:25:49 +00:00
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For chips that require a high voltage VPP pulse during programming, some external
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switching circuitry will be needed to allow the Arduino to control the VPP voltage. There
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is a header connected to the unused D13 pin that can support this. See the
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[Intel 8755 version of the hardware](hardware/#intel-8755a-hardware-version) for an
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example of voltage switching by TommyPROM.
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