BASIC Stamp bugs & gotchas

Contents: (updated 11/18/2000)

• Unwanted resets from noise on the serial port ATN line
• BS2 SLEEP bug, and power consumption in SLEEP mode
• BS2sx, and BS2e sleep bug
• BASIC stamp behavior in brownout
• 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.

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 memory. 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, and by counting the number of gosub's, it can be kept constant by adding extras. 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.

The Sleep Bug, less a factor in the BS2sx & BS2e	top

In the BS2, the effect of the sleep bug is quite dramatic and can result in sleep currents that range from 25 microamps at best up to 450 microamps at worst. This can have a huge effect on the averaged sleep current. On the other hand, for the BS2sx, the sleep current (not averaged) ranges from 46 microamps at best up to about 58 microamps at worst, and has no appreciable effect on the averaged sleep current.

In either the BS2 or the BS2sx, the current can be minimized by padding the instructions before the sleep with a "dummy" instruction that adds just enough bits to move the SLEEP or END instruction up to a favorable position in the eeprom. In the above program the sleep command is placed in a subroutine at the very top of the program code. This does not assure that it stays in one place, because every GOSUB command in the program adds a pointer at the top of memory. Nevertheless, each GOSUB adds exactly 14 bits, and so it is easy to predict the position of the SLEEP command. A value of bitty=63 or a value of bitty=3, one or the other, will always minimize the sleep current. In fact, if the number of GOSUBs, modulo 4, is equal to 1 or 2, then use bitty=63. If it is 3 or 4, then use bitty=3.

The original BS2 is still the champion when it comes to low current operation where the SLEEP instruction is concerned. The BS2 sleeps at an averaged current of around 50 microamps, while the BS2SX sleeps at around 525 microamps. To achieve the lowest current using the BS2, you must take care to position the SLEEP command at a favorable position in the eeprom. With the BS2SX or the BS2e, the position of the SLEEP command in eeprom doesn't make much difference.

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 BS2sx and the BS2e. The original BS2 has an external brownout detector, whereas the BS2sx and BS2e depend on the brownout detector that is built into the SX28 chip. That built-in brownout detector has some 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 BASIC Stamp operates fine down to 4.2 volts, but at that point a brownout detector puts the SX28 chip in 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.
  • 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 suddenly to about 100 microamps. As Vdd drops below 2 volts, the current decreases, until at 1 volt it is about 20 microamps. The current drops precipitously again as Vdd drops below 0.7 volts.
  • The capacitor on the Vdd supply (3.3uF+whatever external capacitance the application circuit adds, if any) discharges in accordence with the current and voltage curve. An initial rapid drop down to 2 volts, due to the higher current, then a gradual taper down to below 1 volt, and then an even more gradual taper down to zero.
  • 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. Recall that these modes consist of very low current sleep periods, punctuated every two seconds by a brief (~18 millisecond) wakeup period where the Stamp checks its internal status to decide whether or not to go back to sleep or to wake up fully to resume execution of a program.
  • Bringing Vdd back above 4.2 volts does not cause a reset, unless Vdd has dropped below 0.6 volts. If the reset does not occur, 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.
  • 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, down to about 6 milliamps at Vdd=2.2 volts. Then at about 2 volts, the current drops to about 100 microamps, and then decreases down to about 20 microamps when Vdd=1 volts. Below that, the current decreases at an ever slower rate attributable to diode leakage characteristics. Note that the current profile is very similar to the one above.
  • If Vdd recovers to >4.2 volts before it sags below about 0.7 or 0.6 volts, the Stamp does not reset. It resumes its sleep-wake or nap or end cycle where it left off.
  • If Vdd does sag below 0.7 or 0.6 volts, then recover to +5 volts, a full reset does occur. The program will execute from the top with all variables zeroed.
• The above results are taken with a BS2e. I expect the results with the BS2sx would be similar, because it too relys on the brownout detector built into the Scenix SX28 chip. The original BASIC Stamp will behave quite differently. It uses the PIC 16C5x chip, with an external brownout detector chip.
• 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 BASIC Stamp 2e or 2sx 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 occured. My observation is that the chip hangs onto its variables quite well, even at low voltages, but it is certainly not guarenteed. The integrity of the variables would be more sensitive to influences of noise (EMF) and extreme temperatures. Another troubling aspect of this behaviour 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.

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