Stretching a bit

I love Design Ideas (DIs) with a backstory.  Recently, frequent DI contributor Jayapal Ramalingam published an engaging tale of engineering ingenuity coping with a design feature requirement added unexpectedly and very (very!) late in product development: “Using a single MCU port pin to drive a multi-digit display.”

Jayapal writes, “Imagine a situation where you have only one port line left out, and you are suddenly required to add a four-digit display.”

Yikes!  Add a looming delivery deadline to build suspense, and this becomes a classic nightmare scenario. It could easily develop, from an engineering standpoint, into a horror story straight out of the pages of Stephen King. Well, okay. Almost.

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But in a clever plot twist, engineer Jayapal shows how a bit (no pun!) of ingenuity turns this tale of terror into an opportunity for some cool circuit design. In his DI, different durations of software-generated pulses on that lonely port line become the control signals necessary for running the newly needed decimal display.

Crisis and calamity averted.

So I wondered how the same basic plot could make a basis for a more generalized storyline. In this version, not just four digits of numerical binary-coded decimal (BCD), but N bits of arbitrary parallel binary outputs would be driven in a similar solitary serial fashion. And all this would be achieved by the same singleton GPIO port bit. Figure 1 shows how the story takes shape.

Figure 1 A lonely GPIO bit loads a lengthy serial string of parallel registers. 

Incoming pulses of variable length on GPIO are buffered by noninverting gate U1a and drive three sets of inputs. 

1. Timing circuits U1b (400us R1C3 SER input zero/one discriminator),
2. U1cd (2.4ms R4C2 parallel RCLK clock AC coupled Schmidt trigger),
3. SRCLK shift registers serial clock.

As illustrated in Figure 2, the interpulse (idle) state of the GPIO is high = 1. 

Figure 2 GPIO pulse timing.

A serial bit transfer pulse starts when the GPIO goes low = 0, releasing the timing RCs. Whether the pulse shifts to a 0 or 1 bit depends on its duration.  If < 100 μs (T0), the R1C3 timeconstant will still hold SER low when the rising edge of SRCLK clocks the serial registers. This will cause a 0 bit to be shifted in. If > 400 μs (T1), the opposite will occur, and the shift register gets a one.

In this way, a data rate between 2 kbps and 10 kbps (depending on the relative frequencies of ones and zeros) can be maintained as long as the idle period between pulses remains less than 600 μs. Completion of data transfer is signaled by allowing GPIO to remain idle for > TR = 3.5 ms.  This allows R4C2 to time out and a transfer pulse to occur on RCLK, commanding a broadside parallel data transfer from the shift registers to the parallel output bits.

Note that, going back to the original horror story, four BCD digits = 16 bits, two 8-bit shift registers, and 12 ms would be enough logic and time. I think that makes for a pretty good ending for a yarn about a far stretch of a single bit.

Stephen Woodward‘s relationship with EDN’s DI column goes back quite a long way. Over 200 submissions have been accepted since his first contribution back in 1974.  They have included best Design Idea of the year in 1974 and 2001.

Related Content

• Using a single MCU port pin to drive a multi-digit display
• Silly simple supply sequencing
• Short push, long push for sequential operation of multiple power supplies
• How to get the most out of a single timer on an MCU

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