Motorola 68HC11

Microcontroller MC68HC11A8.

The Motorola 68HC11 family (abbreviated HC11 or 6811) is a family of Motorola microcontrollers, derived from the Motorola 6800 microprocessor. The 68HC11 microcontrollers are more powerful and more expensive than the 68HC05 family and are used in multiple embedded systems.

They follow the von Neumann Architecture, in which program, data, and input/output memory are addressed in a single memory map.

Internally, the instruction set of the 68HC11 family is compatible with that of the Motorola 6801 and Motorola 6809, with the addition of a Y register (which can be used by the same instructions as the X register). The 68HC11 family employs variable-length instructions and is considered to employ a CISC architecture. They have two eight-bit accumulators (A and B), they have a virtual accumulator D, which is nothing more than the union of A and B (16 bits), two 16-bit index registers (X and Y), a flag register, a stack pointer, and a program counter.

The Freescale 68HC12 microcontroller is an enhanced 16-bit version of the 68HC11.

The Freescale 68HC16 microcontroller is a mostly software compatible 16-bit version of the 68HC11.

Fred Martin's Handy Board robotic controller is based on the 68HC11.

The 68HC11 has five external ports (A, B, C, D, and E), each eight bits except D, which is typically six bits.

Port A is used for event capture, output comparison, pulse accumulator, and other clock functions; port D for serial I/O and port E as an analog-to-digital converter.

The 68HC11 family can work with either internal or external memory. In case of using external memory, ports B and C work as data bus and addresses respectively.

Doors

It must be said that the ports can function as general purpose digital input-output terminals. However, they can be configured to perform a specific function, associated with each port. The alternate function of each port is detailed below. (The bits are numbered starting at zero.)

Block diagram for MC68HC11A8.

PORT A (Clock and timing functions).

It consists of 8 pins, three of which are input (bits 0-2), four output (bits 3-6) and one bidirectional (bit 7).

Bits 0-2: Input grabbers. Every time the programmed event occurs on one of these three pins, the content of the TCNT register is copied to another TICx register, where x will be 1,2 or 3, according to the pin that we treat. The event to be recorded can be configured and can be a rising edge, a falling edge or a pulse at a high level.

Bits 3-6: Output comparators. It uses a series of registers (called TOCx) in which we write a numerical value. When this value matches that of the TCNT register, an event will be produced on the corresponding pin of the port. It is also configurable and can be programmed to toggle, toggle low, or toggle high.

Bit 7: Comparator/Accumulator. As has been said, this pin is bidirectional, so when it is configured as an output it functions as an output comparator, but this time it can also act on the other output pins (bits 3-6). If configured as an input, it works as a pulse counter. Through registers, it can be programmed to count each time certain events occur, such as falling edge, rising edge, high level or low level.

PORTS B and C (Expansion Bus).

The 68HC11 has a memory map made up of the memories it has internally. However, out of the entire range of possible addresses, most are idle. The microcontroller can be configured in a special mode (expanded) which uses ports B and C to address all possible memory locations (namely 16 bit addresses <-> 65535 locations).

Port B is used to place the most significant bits of the addresses (Bits 8-15).

Port C has dual use: least significant bits of the address bus (0-7) and data bus (8 bits). To do this, a signal called AS is used, which when activated indicates that port C is setting a valid address. Then it changes, and places the data bits. To be able to do this, an external circuit must be added, specifically an 8-bit enable register, so that it captures the address when AS is active and holds it when it is deactivated. Normally it is usually the 74HC573.

PORT D (Serial communication).

Synchronous and asynchronous serial communication protocols can be established.

Asynchronous Serial Port (SCI). There are two devices, where the output of one is connected to the input of the other, and vice versa. The clock signal has the same frequency for the two devices, but it is not shared by them.

Bit 0 Reception (Rx). Input of the data bits.

Bit 1 Transmission (Tx). Output for the data bits.

Asynchronous serial communication is often used in systems that are several meters apart. Because the transmitter can send bits at any time, it is necessary to use a protocol that allows knowing where the beginning of the data is and where its end is (beginning btis, end...). The transmission speed is measured in Bauds (Baud), a value of 9600 Bauds being normal. All of these parameters are configurable via registers in the 68HC11.

Synchronous Serial Port (SPI). In this type of communication there is a device that behaves as a master and one or more that are subject to it and are called slaves. A clock signal is used that will be the same for all devices. Four bits are used:

Bit 2 MISO (master input-slave output). Data output from a slave to the input of the master.

Bit 3 MOSI (in English, master output-slave input). Data output from the master to the slave(s).

Bit 4 SCK, clock signal.

Bit 5 /SS, allows you to select which slave the master wants to communicate with.

This communication is typical of peripherals that require high transmission speeds, although these can only be separated by several centimeters. As the clock signal is shared, it is not necessary to add additional start or end bits, and the transfer rate increases. Clock configuration, masters, slaves, etc. it is done by register programming.

PORT E (Analog-digital conversion, CAD).

The 68HC11 consists of an analog/digital converter, which receives a voltage between 0 and 5 volts; and returns, via internal registers, 8-bit values (0 to 255), which are proportional to the input. These values can be used in the program to make calculations, for example.

If the HC11 model is 52-pin, the port will have eight inputs to the converter, while if it is the 48-pin model, this port will only have four inputs.

There are several ways to work with CAD. One is to perform conversions of: either the four least significant bits of the port (bits 0-3), or the four most significant bits (bits 4-7). Another way is to select a single pin and work with it.

In addition, the CAD can be configured to convert once, or to perform continuous conversions.