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idea about the hardware design points associated with the LPC2106. However, we’re also in the early ■ FIGURE 2. The top components shown in this figure are pretty much the same as what you saw in Figure 1 for the LPC2106. The difference in the LPC2106 and the LPC2136 are evident in the peripheral area. I particularly like the analog-to-digital component. design stage for another Philips ARM7 variant — the LPC2136. THE LPC2136 One of the really neat things about Philips LPC21XX ARM7 microcontrollers is that you can transfer knowledge that pertains to one of the LPC2100 variants to any of the other members of the LPC2100 family. Everything you’ve learned about the LPC2106 can be applied to the LPC2136. Both the LPC2106 and the LPC2136 are based on the same ARM7TDMI-S CPU and the 128-bit wide memory interface and accelerator architecture are also present within 88 April 2006
■ PHOTO 1. This is a bird’s eye view of the LPC2106 adapter board. Note the three .1 μF bypass capacitors mounted on the bottom side. the LPC2136. In fact, the only major differences between the LPC2106 and the LPC2136 are the number of peripherals and the power requirements. Figure 2 shows us that the LPC2136 is a larger microcontroller in terms of peripherals, SRAM, and Flash storage. The LPC2136 isn’t the Big Daddy here and has a big brother — the LPC2138 — which carries 512KB of Flash and 32KB of SRAM. The LPC2136 matches the LPC2138’s SRAM size, but only houses half of the LPC2138’s available Flash program memory. The give and take between the LPC2106 and the LPC2136 lies in the LPC2106’s larger SRAM size and reduced peripheral count versus the LPC2136’s smaller amount of available SRAM and increased peripheral count. This offset of SRAM versus peripheral count correlates with the first rule of embedded computing which states that nothing is free. If you want more peripherals, you must give up something else. In this case, that something else is SRAM. As you can see in the LPC2106, the first rule of embedded computing works in the opposite way as peripherals are sacrificed for additional SRAM. This give and take is a good thing, as it produces multiple variants of the LPC210X and LPC213X microcontrollers, which allows for more cost effective designs. The LPC2106 is a 48-pin device. Since Figure 2 shows us that the LPC2136 has 16 more general-purpose I/O pins and pairs of I 2 C, SPI, and analog-to-digital converter interfaces, it would be logical to deduce that the LPC2136 is a physically larger part. The extra peripheral general I/O interfaces push the LPC2136’s pin count to 64. Note, also, that the LPC2136 supports an on-chip digital-to-analog converter subsystem. Another advanced feature of the LPC2136 is the battery-backup capable real time clock. Many of the hardware design points that exist for the LPC2106 are common to the LPC2136. However, we must take the battery-backable real time clock of the LPC2136 into account if we plan to use it. Also, there is no need for a +1.8 VDC power source for the LPC2136, as it only requires a +3.3 VDC power source. We’ll get to know the LPC2106 and the LPC2136 internals better when we begin the coding phases of our ARM7 designs. So, let’s attack the hardware design points and build up some ARM7 hardware. FABRICATING ARM7 MICROCONTROLLER SYSTEMS THE DESIGN CYCLE We won’t be using custom printed circuit boards at the system level in this installment, as you may want to create your own flavor of the LPC21XX designs I’m about to present to you. With that thought in mind, I’ve decided to use some prototype boards from other EDTP products that contain +3.3 VDC power supplies and ready-to-go RS-232 serial ports. Once we’ve verified the basic operations of the ARM7 parts, I’ll put the final designs down onto some professional printed circuit boards, which I will make available to you via the EDTP Electronics website. Now that I’ve chosen to use cast-away prototype boards, the first obstacle we have to negotiate is how we will mount the fine-pitched LPC21XX devices. The LPC2106 is tiny and its 48 pins are positioned equally around seven square millimeters. The LPC2136 is equally difficult to breadboard, as it is not much larger with its 64 pins being ■ PHOTO 2. Here’s a shot of the LPC2136 adapter board top and bottom. The far right bottom capacitor is an 0805 SMT 10 μF at 6.3V tantalum that is filtering the Vref pin. The rest of the capacitors are .1 μF 0805 SMT devices, which are acting in the power supply bypass role. April 2006 89
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