BASIC Stamp bugs & gotchas

Contents: (updated 11/18/2000, 2/20/2009)

• Unwanted resets from noise on the serial port ATN line
• BS2 SLEEP bug, and power consumption in SLEEP mode
• BASIC stamp behavior in brownout
• Sub-zero temperature failures in SX-based Stamps
  
• Serial port timing bug

BS2 sleep bug	top

The BS2 is advertised to have a current drain of less than 50µamps in sleep mode, and experience has shown that current production units are running a best sleep current of around 30 µamps. However, there is a subtle bug in the BS2 memory circuits, which can result in a sleep current of as much as 350 microamps. It depends on exactly where in memory the SLEEP instruction is encountered by the interpreter. The SLEEP instruction has to fall at a "good" position in the eeprom or else the sleep current drain will be greater than 300 microamps instead of less than 50 microamps. If you just let it fall where it may, you have something like a 50-50 chance for it to fall in a good position. In some applications, such as remote data logging, the system can spend a great proportion of its time in sleep mode, and the sleep current is a major factor in the battery life. It is better not to leave that to chance.

NOTE (2008): The EEPROM currently used on the most recent Stamp I looked at had much better performance.  At worst the SLEEP current was 58 microamps instead of 350, and the best SLEEP current was 17 microamps.   However, there is still a caveat.  This note applies to all of the Stamps, but to what degree depends on the interpreter EEPROM that is used on the Stamp module.   The EEPROMs from different manufacturer's have different characteristics, and through time the manufacturer's make changes as they upgrade their manufacturing processes, often with no notice or change in part number.   The Stamp uses the EEPROM in an "off-the-books" manner, by addressing it one bit at a time instead of one byte at a time, so the SLEEP instruction can leave it active at an address that is not on a byte boundary.   "Off-book" means that the specifications for quiescent current drain in the data sheets may not apply.   It this  issue is of concern for your application, I suggest that you do the experiments yourself.   This note continues below after my comments about the workaround.

I had thought to put my one SLEEP instruction near the beginning of my programs, so as to fix its position in the eeprom for once and for all to minimize current. But that didn't work as easily as planned, because each GOSUB instruction adds code at the top, above the program in eeprom. In fact, each GOSUB adds exactly 14 bits at the top of the program. Nevertheless, it is still worthwhile to put the SLEEP instruction in a routine near the top of the program. Then the adjustment only needs to be made when the total number of GOSUBS in the program changes. Here's a skeleton of a program, to demonstrate how the sleep routine fits in near the top, and how the exact position of the SLEEP command is adjusted:

zb var bit ' bit variable for pre-sleep adjustment
well var byte ' sleep duration
' define other variables/constants/data

start:
goto main0 'skip over the bed time code

bed: ' bed time subroutine
' it goes here to fix it near the top in eeprom
zb=1 ' play with zb to minimize current
' case 0) use zb=15
' case 1) use zb=1
' case 2 & 3) comment it out ' zb=.
' see below for explanation
sleep well ' no driven loads! no floating inputs!
return

main0:
' initializations
main:
' program goes here
well = 10 ' how long to sleep
' no driven outputs, no floating inputs!
' application dependent
gosub bed ' do it, and return here
' more program
goto main
end
'gosub bed
'gosub bed
'gosub bed

Here's the trick, hardware and thinkware versions:

1) Measure the stamp current during sleep. Try zb=1, zb=15 or comment out the zb= statement. One of these 3 possibilities will result in <50 microamps current. (Assuming no driven loads, floating inputs etc.) If the current is around 300 microamps, try another possibility.

or

2) Count the number of gosubs in the program, divide by 4 and look at the remainder.

• Remainder is 0, then use zb=15.
• Remainder is 1, then use zb=1.
• Remainder is 2 or 3, then comment out the zb= statement.

That's it. The above program as it stands has one gosub, so zb=1.

How to count gosubs? Use the ALT-F find "gosub"; ALT-N again and again; keep a mental tally. Don't count remarks of course! The FIND function in the BS2 editor is case sensitive. If you're not sure that all of them are cased the same, you might want to use a more capable text editor to do the count.

As a practical matter, there are installations where it is difficult to measure the sleep current, so counting gosubs is an attractive alternative. I wish the bug would just go away! The above method of counting GOSUBs may not always work. I tried it on some shipments of BS2s and it did not work as expected. But the business of measuring the sleep current, and trying either zb=15. zb=1 or no zb at all was always able to yield the minimum sleep current.

Note that this workaround is effective only if the BS2 has the Microchip EEPROM. For a short period of time, Parallax was shipping BS2s with an ATMEL EEPROM, for which the above workaround is not effective at all. Parallax stopped using the ATMEL parts as soon as the problem was discovered.

As noted above, the effect depends on the EEPROM, and applies across the Stamp family.   As an example, consider the Catalyst EEPROMS used originally on the BS2pe.   The current drawn by the EEPROM itself was 3 to 15 microamps in the original CAT24WC256KI, (chip marked CSI  24WC256KI), but that was discontinued, and their most recent version is the CAT24C256XI, and in the ones I tested the current was 75 microamps minimum to 1800 microamps maximum.    Quite bad.   I was evaluating the chips for my OWL2pe data logger, where low SLEEP current is essential.   I settled on the Microchip EEPROM, 24LC256I/SM, which gives a current range from 20 to 190 microamps.   In micropower situations, I still have to align the SLEEP instructions, but for most applications, even 190 microamps is not too bad.   Again, the messages is, if it is important to you, measure it. 

In terms of overall current consumption, the original BS2 is great.   Current versions are less than 3 milliamps of operating current and 20 microamps during SLEEP.    The BS2pe (as implemented on the OWL2pe data logger) draws about 16 milliamps when operating and 70 microamps while asleep.   The advantage that the BS2pe has is its shorter wakeup intervals while SLEEPing, 0.15 millisecond versus 15 milliseconds.    See my article on the BS2pe.

Brownout behavior, a bug?	top

BASIC Stamps operate with a Vdd power supply of nominally 5.0 volts, but they can operate on slightly higher and slightly lower voltages. What is of concern here is low voltages as might come about, say, as a battery voltage dips in a solar charge cycle, or in a system that occasionally has brief power failures. Sags in voltage are called brownouts, while a complete and sustained loss of power is called a blackout.

The following discussion applies only to the Stamps that rely on the brownout detector that is internal to the SX28 or SX48 microprocessor chip. The original BS2 has an external brownout detector, so this note does not apply to it.   Pre-2008, all of the multislot BASIC Stamps relied on the internal brownout detector, but in 2008, Parallax revised the BS2e (rev E) and BS2sx (rev F) hardware to include an external brownout detector, and in the future may also do so with the BS2p family, but as of this writing, the BS2p family still relies on the internal brownout detector. 

That SX  brownout detector has characteristics that can only be described as a "bug". But it is only a bug if you are using the SLEEP, NAP or END instructions and expect your system to behave well in those low-power modes as the battery voltage drops.

The affected BASIC Stamp operates fine down to 4.2 volts, but at that point a brownout detector puts the SX chip into a reset state. What happens at that point depends on what the Stamp was doing when it hits the brownout threshold, and on how low the voltage dips. Here are a few observations:

• If the Stamp was operating (not executing SLEEP, NAP or END) when Vdd drops below the 4.2 volt brownout threshold...
  • The Stamp stops executing the program.
  • Bringing the power back up above 4.2 volts causes a reset, just as if you had pressed the reset button. This is true no matter how low the voltage drops. The variables are all initialized to zero, and the program executes from the top.  This is what you would expect to happen.
    
  • The current drain of the BS2e is near 12 milliamps when Vdd is just less than 4.2 volts, and drops to about 6 milliamps when Vdd is just above 2 volts. At about 2 volts, the current drops sto about 100 microamps and decreases as the voltage drops further.
    
  • Restoring the power to Vdd>4.2 at any time, either suddenly or gradually will reset the chip and the program will execute from the top with all variables zeroed.
• If the Stamp was executing SLEEP, NAP or END when Vdd drops below the 4.2 volt brownout threshold, but stays above about 0.7 volts...
  • Bringing Vdd back above 4.2 volts does not cause a reset, and it resumes its sleep-wake or nap or end cycle where it left off.
  • The sleep or nap cycle resumes where it left off. For example, if there were 10 seconds left in a sleep cycle when the chip dropped below the brownout threshold, those 10 seconds are resumed after the recovery from brownout. The time spent in brownout does not count.
  • When the chip first drops below the 4.2 volt threshold, the current drain is about 35 microamps, but when the first watchdog timer even occurs, the chip wakes up into a never-never land brownout condition. The current jumps up to about 12 milliamps.
  • As Vdd drops lower, the current also drops, to about 100 microamps at Vdd=2 volts and 20 microamps at Vdd=1 volts.
• The same but if Vdd drops below about 0.6 volts, which takes some time as the power supply capacitors need to discharge...
• The Stamp does a full reset, the variables are zeroed, and program execution starts from the top.
  
• The original BASIC Stamp will behave quite differently. It uses the PIC 16C5x chip, with an external brownout detector chip.  Likewise the most recent (2008) models of the BS2e (rev E) and BS2sx (rev F) modules.
  
• I have also tested the Scenix SX18 chip in a different circuit, with the watchdog disabled. That makes quite a difference in the current drain during the brownout condition. The current stays at about 35 microamps or less as Vdd drops. The current tapers off ever more slowly as Vdd decreases, especially below 0.7 volt. The SX18 does not execute a complete reset when it recovers from brownout, unless Vdd has fallen below the 0.6 volt mark that causes a Power-On reset condition. This contrasts sharply with the behavior in its normal operating mode, where a full reset will occur after any brownout.
• It seems to me that this behavior is a bug in the Scenix chip. It should perform a full reset upon recovery from any brownout event.

What of it? The troubling thing for the affected BASIC Stamp is that the chip does not reset as a result of brownouts that occur during sleep intervals. There is a possibility that variables would be corrupted during the brownout, and there would not be a clue as to why that had occurred. My observation is that the chip hangs onto its variables quite well, even at low voltages, but it is certainly not guaranteed. The integrity of the variables would be more sensitive to influences of noise (EMF) and extreme temperatures. Another troubling aspect of this behavior is that the current drain becomes quite high if the chip goes into brownout. Usually a system uses the SLEEP mode to conserve battery power, but the higher current in brownout could deal a final blow to a marginal battery.   On my OWL2pe processor, the battery condition is monitored and when it is critical, the firmware puts the logger into a hibernation mode in order to forestall any pending brownout condition.

Failure at sub-zero temperatures in SX chips at brownout setting BOR42	top

The SX internal brownout detector set at 4.2 volts has a serious problem at low temperatures, and will go into never-never land at a certain temperature that for different units will be in the range of -5 to -25 degrees Celsius, and typically at -15.     By never-never land, I mean that the SX clock oscillator keeps running, the current consumption jumps, and the chip stops executing code.   This was a serious problem for my early OWL2pe data loggers when they had to operate in outdoor winter environments down to as low as -40 degrees Celsius.    Using an environmental chamber, I traced the problem to the BOR42 setting on the SX chip, and was able to resolve the problem by using a special version of the BS2pe chip that has a setting of BOR26.   With that setting it operates without fail in tests down to -65 degrees Celsius, well below the -40 degree Celsius component ratings on the OWL2pe.    Parallax has resolved the problem as of 2008 on the BS2e (rev E) and BS2sx (rev F) modules by a hardware revision to an external reset chip, and has extended the operating temperature range -40 to  +85 degC  for those modules.

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