The ST-2900 NVRAM board is a small expansion board that plugs into the ST-2900 FDC board to provide non-volatile SRAM-Disk and EEPROM-Disk functions. The RTC module is a small daughterboard that optionally plugs into the NVRAM board to add a real-time clock (RTC).
NVRAM and RTC board features:
Comparison to old RAM-512 board:
The Micro Crystal RV-3149-C3 RTC is temperature compensated (with a TCXO), so typically loses or gains less than 8 seconds per month. The surface mount RTC and supporting devices are installed on a separate daughterboard in the form of an 8-pin DIP module that plugs into one of the SRAM sockets. The RV-3149-C3 also has a built-in charging circuit that can be enabled to allow using a supercapacitor instead of a lithium cell to provide backup power to the RTC and SRAM.
(Another way to add a real-time clock to the ST-2900 will be with the RTC-2417 board.)
The driver software divides the SRAM and EEPROM sections of the NVRAM board into FLEX and (Nitr)OS-9 partitions. A planned future revision to ST-MON will allow it to boot FLEX and (Nitr)OS-9 from these partitions, eliminating the need to have any floppy disk drives attached to the ST-2900.
An SPI-compatible (Serial Peripheral Interface) bus connects the SRAM, EEPROM, and RTC devices to port B of the 6522/VIA on the FDC board. Although the 6522's shift register can't implement a full-duplex SPI host, all the slave devices on the NVRAM/RTC board only communicate in one direction at a time, using either the MOSI (SI/D/DIN/DQ0) or MISO (SO/Q/DOUT/DQ1) lines, but not both at the same time, so this works. Since the 6522's shift register does not directly support SPI Mode 0 (0,0), which is required for the RV-3149-C3 clock chip, some helper circuitry was added. The 6522 runs the SPI bus at 500kbps when the system clock is 1MHz.
It is possible to assemble a subset of this board with only easy-to-solder thru-hole components. This limits you to a 1MB SRAM-Disk and 256KB EEPROM-Disk (using the 25LC1024 EEPROM in a DIP-8 package). The larger capacity EEPROM chips and the RTC and its supporting devices are all surface mount.
This board could also be used with any other system that has a 6522/VIA with all of port B unused. If the RTC module is installed, the system clock must be 2MHz or slower.
(Click on a picture for a larger version.)
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| Bare NVRAM and RTC boards. (114KB-1.1MB .jpg) | ||||
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| Partially assembled NVRAM board, development setup with EEPROM 7 Click board and RV-3149-C3 eval board, and NVRAM spacer boards. (272-444KB .jpg) | ||||
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| Fully assembled NVRAM board. (??-??KB .jpg) | . | |||
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| Fully assembled RTC module. (??-??KB .jpg) | ||||
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Gerber files for manufacturing ST-2900 NVRAM and RTC circuit
boards. (not available yet) (??KB .zip, last modified 2023-Aug-26) |
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ST-2900 NVRAM/RTC Hardware and Software User's Manual
(for FLEX and (Nitr)OS-9 Level 1). (not available yet) (??KB .pdf, last modified 2023-Aug-26) |
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DSK image (35T/SS/SD FLEX format) of source code and binaries of
FLEX Plus 10TM
drivers and supporting utilities for ST-2900 NVRAM/RTC board for SRAM-Disk,
EEPROM-Disk, and clock. (not available yet) (??KB .zip, last modified 2023-Aug-26) |
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DSK image (35T/SS/DD CoCo OS-9 format) of source code and binaries
of (Nitr)OS-9 Level 1 drivers and supporting utilities for ST-2900
NVRAM/RTC board for SRAM-Disk, EEPROM-Disk, and clock. (not available yet) (??KB .zip, last modified 2023-Aug-26) |
Current Status (updated 2023-Feb-22)
I have partially assembled one board (see picture above). Capturing the SPI signals with a logic analyzer shows that the SPI Mode 0 (0,0) helper circuitry is working as intended for both writes and reads. Yes, the 6522/VIA's shift register can support half duplex SPI without needing to resort to bit banging!
I am successfully writing to and reading from one SRAM chip, two EEPROM chips (25LC1024 and 25CSM04), and the RTC chip, and blinking the activity LED. A preliminary version of the patches to ST-MON to boot FLEX and OS-9 from all 3 memory chips has been written and is working.
The OS-9 driver has been written and briefly tested with all 4 chips, and appears to be working correctly. I have written the disk image format program, a program to check and display the error correction status of all sectors in the 25CSM04 EEPROM chip, and a program to set the date/time in the RTC chip.
The FLEX driver has been written and briefly tested with 3 of the 4 chips (not the RTC), and appears to be working correctly. It is compatible with the new FLEX Plus 10TM Driver Executive. I have also written the disk image format program, but have not yet written the program to check and display the error correction status of all sectors in the 25CSM04 EEPROM chip, nor the program to set the date/time in the RTC chip.
The 25CSM04 EEPROM was out-of-stock at all the distributors, and even at Microchip, with long lead times. I placed my order 2021-Jul-15, but the estimated shipping date was updated (delayed) 32 times! After cancelling my order for the cut tape packaging, and re-ordering the tube packaging, I finally received the EEPROMs on 2023-Feb-01 -- only 18.5 months late! Now I can assemble two full NVRAM/RTC boards.
But while waiting for them I was able to purchase one Mikroe EEPROM 7 Click board from Digi-Key, and have connected it to the NVRAM/RTC board with an IDC DIP header and ribbon cable (see picture above). As a bonus I didn't need to find someone to solder the SOIC chip to my board (since I can't solder surface mount chips).
I also connected an RV-3149-C3 evaluation board for the RTC chip to the NVRAM/RTC board using an IDC DIP header and ribbon cable (see picture above). I was able to purchase the chip itself, but it is tricky to solder, so the eval board eliminated that difficulty for now.
Another ribbon cable connects the NVRAM/RTC board to P2 on the FDC board for easier access to the NVRAM/RTC board during development.
I needed some spacer PCBs for the 14-pin connector. Instead of trying to remember how to use Easy-PC, or learning to use KiCad, I just used a text editor to manually create the Gerber and drill files. I ordered the boards from OSH Park -- they cost U$2.75 with free shipping!
When I powered up the ST-2900 on 2022-Jun-20, it hadn't been powered for 114 days. Running on battery power during this time, the RTC was now 28 seconds fast, or just over 7 seconds/month (2.7 ppm error). This is good considering I have the less accurate B option RTC with +/-25ppm rated accuracy (over the -40°C to +85°C range). Option A is rated at +/-6ppm, although at a constant 25°C they are both rated at the same +/-3ppm.
Work continues on the supporting software.
"FLEX" was a trademark of Technical System Consultants (TSC).
"FLEX Plus 10" is a trademark of David C. Wiens, dba Sardis Technologies.
"OS-9" is a registered trademark of Microware LP.
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Last revised 2023-Sep-08 13:26 PDT.
Copyright 2018-2023 David C. Wiens.
