sergey
Veteran Member
Hi,
A few people asked recently about an RTC card for PC/XT. I am wondering if it is something worth designing?
Now to the technical aspect:
I looked into several design options. Here are some of them:
1. Use Maxim/Dallas DS1501/DS1511 chip.
Pros:
- Chip is currently in production. The DIP version is not recommended for new designs, but it is possible to design a PCB that will accommodate both DIP and SOIC packages.
- Chip has "century" byte which simplifies the Y2K compatibility.
- Chip has convenient I/O implementation (8-bit data + 5 address lines), and requires minimal number of additional components to interface to the XT/ISA bus. (5-bit address / 32 address locations is a bit overkill for scarce XT address space, although it is possible to ignore NVRAM capability, and reduce number of address locations to 16).
Cons:
- The chip is a bit expensive - about $10 in single quantities. Add $10 or so for PCB and $5 for other components... So $25 per board without labor.
- Will require writing a driver (but it isn't that complicated)
2. Use MM58167 chip.
Pros:
- Chip is used in many older RTC cards. The design might be compatible with one of them.
Cons:
- Chip is out of production (although available through various Chinese IC dealers).
- Chip doesn't support years count. So driver needs to take care of figuring out the year (e.g. storing it in NVRAM), and adjusting for leap year.
3. Use DS12885 (DS1287/DS12887) chip
Pros:
- Used in AT
- Currently in production.
- There is DS12R885 version that support trickle charge, and can be used with supercaps instead of regular batteries. It is also cheaper ($5/chip), but unfortunately only comes in SOIC package.
Cons:
- No century count / two digit year (excluding DS12887C variant). Century will need to be maintained by driver, or alternatively it can be assumed that it will not be used beyond 2080
- A bit more cumbersome interface to ISA (it has a multiplexed address/data, so additional logic is required for interfacing)
- Will require writing a driver
4. Use one of DS1307 compatible I2C chips.
Pros:
- Generally cheaper than chips with parallel interface.
- High precision models are available, e.g DS3231M with built-in MEMS resonator (although at a bit higher price)
Cons:
- Will require something to interface to the parallel ISA bus. Anything from bit banging interface (a latch and a buffer) to a microcontroller with I2C and parallel interfaces (e.g PIC16F877). In the latter case it is possible to have some additional functionality (e.g. an SD-card interface)
- Will require writing a driver
Thanks,
Sergey
A few people asked recently about an RTC card for PC/XT. I am wondering if it is something worth designing?
Now to the technical aspect:
I looked into several design options. Here are some of them:
1. Use Maxim/Dallas DS1501/DS1511 chip.
Pros:
- Chip is currently in production. The DIP version is not recommended for new designs, but it is possible to design a PCB that will accommodate both DIP and SOIC packages.
- Chip has "century" byte which simplifies the Y2K compatibility.
- Chip has convenient I/O implementation (8-bit data + 5 address lines), and requires minimal number of additional components to interface to the XT/ISA bus. (5-bit address / 32 address locations is a bit overkill for scarce XT address space, although it is possible to ignore NVRAM capability, and reduce number of address locations to 16).
Cons:
- The chip is a bit expensive - about $10 in single quantities. Add $10 or so for PCB and $5 for other components... So $25 per board without labor.
- Will require writing a driver (but it isn't that complicated)
2. Use MM58167 chip.
Pros:
- Chip is used in many older RTC cards. The design might be compatible with one of them.
Cons:
- Chip is out of production (although available through various Chinese IC dealers).
- Chip doesn't support years count. So driver needs to take care of figuring out the year (e.g. storing it in NVRAM), and adjusting for leap year.
3. Use DS12885 (DS1287/DS12887) chip
Pros:
- Used in AT
- Currently in production.
- There is DS12R885 version that support trickle charge, and can be used with supercaps instead of regular batteries. It is also cheaper ($5/chip), but unfortunately only comes in SOIC package.
Cons:
- No century count / two digit year (excluding DS12887C variant). Century will need to be maintained by driver, or alternatively it can be assumed that it will not be used beyond 2080
- A bit more cumbersome interface to ISA (it has a multiplexed address/data, so additional logic is required for interfacing)
- Will require writing a driver
4. Use one of DS1307 compatible I2C chips.
Pros:
- Generally cheaper than chips with parallel interface.
- High precision models are available, e.g DS3231M with built-in MEMS resonator (although at a bit higher price)
Cons:
- Will require something to interface to the parallel ISA bus. Anything from bit banging interface (a latch and a buffer) to a microcontroller with I2C and parallel interfaces (e.g PIC16F877). In the latter case it is possible to have some additional functionality (e.g. an SD-card interface)
- Will require writing a driver
Thanks,
Sergey
