September 1998, Issue 98

PIC'Spectrum
Audio Spectrum Analyzer

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YOU SAY PIC17C756?

Before getting into more details about PIC’Spectrum (see Photo 1), let’s have a look at the PIC17C756 and its stripped-down version, the PIC17C752. These new fully static CMOS chips are an enhancement of the existing PIC17C4X family.

Photo 1—The PIC’Spectrum prototype is built on a 1" × 3" PCB. A good ground plate is mandatory on a mixed digital/analog design like this one. I used a transparent box to prove that there are very few components inside.

The microcontroller core runs up to 33 MHz, giving a 121-ns instruction cycle. These chips are pure RISC design, and thanks to the two-stage pipeline, each instruction executes in only one cycle, except program branches and table reads/writes, which are two cycles long. That makes it near 8 MIPS—not bad for a microcontroller.

The 58 single-word instructions (coded on 16 bits) are easy to learn for any user of Microchip’s smaller controllers. Direct, indirect, and relative addressing modes are supported. External interrupts are present, as is a 16-level hardware stack.

One interesting feature for compute-intensive applications: there is an integrated 8 × 8 bit hardware multiplier, working in only one instruction cycle. This multiplier offers a performance boost of more than three times (compared to a software-only version) for a complete FFT calculation.

The internal memory is impressive as well. The PIC17C756 has 16 K words of EPROM program memory as well as 902 bytes of general-purpose RAM, which accommodates quite large projects without the need of external memories, even if extended modes are available and support up to 64 K words of program memory.

The smaller PIC17C752 has only 8 K words of EPROM and 454 bytes of RAM. Microchip has also announced flash-memory versions (PIC17F75X). The memory map is shown in Figure 2.

Figure 2—From this memory map of the PIC17C756, you see that the program and data areas are separated, as usual on a Harvard architecture. The ’17C752 has the same memory map, except that there is only 8 KB of user program memory and two user RAM banks.

One piece of bad news: the RAM and registers are still banked, and the working page is selected by some bits in the BSR register. Even if some specific assembler instructions help, this is a major headache for the programmer and a major source of bugs. I hope that Microchip switches to a linear address mode in the near future.

Hosted in 64- (DIP) or 68-pin packages (PLCC and TQFP), the PIC17C75X provides 50 I/O pins with individual direction control. As usual, each pin may be used for general-purpose I/O or dedicated to some on-chip peripherals.

Have a look of the pinout of the PLCC version in Figure 3 and you’ll understand that this chip is quite flexible. Both OTP and windowed versions exist, even if they aren’t so easy to find.

On the peripheral feature list, there are four timers (two 16 bits wide and two 8 bits wide, TMR0 having an internal 8-bit programmable prescaler), four capture input pins, and three PWM outputs with a 10-bit resolution.

Need more? Perhaps two asynchronous and one synchronous serial port, the latter configurable both in SPI and I2C modes, master or slave? Or a 12-channel 10-bit ADC? Or an RC-clocked watchdog timer?

How about an integrated supply-voltage supervision? Or a configurable RC/crystal/ceramic clock system with an oscillator start-up timer? Name it, and it’s probably there.

The PIC17C756X also has in-circuit programming hardware. By pulling the TEST and MCLR/VPP pins to a 13-V programming voltage before powering up the chip, a specific ROM bootstrap code is launched and waits for orders coming from a dedicated serial synchronous interface (RA1/RA4/RA5 pins).

With a small interface connected to the LPT port of your favorite computer, you can read or write the internal EPROM without any specific programmer and without having to extract the chip from its socket.

I developed my own PC-based in-circuit programmer based on Microchip’s programming specifications [2].

Electrically, the standard-grade PIC17C756 needs a supply voltage from 4.5 to 6.0 V, while a special extended device (PIC17LC756) accepts anything from 3.0 to 6.0 V. In both cases, the minimum RAM retention voltage is 1.5 V.

The supply current is 6 mA at 4 MHz but climbs to 50 mA at 33 MHz. Of course, sleep modes reduce consumption down to 1 µA, depending on the selected peripherals, but high performances and low consumption are still difficult to conciliate.